Confusion about Bitcoin Core

[DarkState]

I have some confusion about Bitcoin Core. I want to clear these up a bit.

1. I have Bitcoin Core version 30.0 installed on my PC. But Syncing Headers is still 25% complete.
Will I be able to make Bitcoin transactions even if it is not fully completed?

2. If my PC ever crashes, if I have to reinstall Windows, and later install Bitcoin Core again,
how will I get my previous wallet back? I haven't seen any authentication login option in Bitcoin Core.

3. Suppose I have 5 members in my family. We only have one PC. So, if everyone in the family wants to use Bitcoin Core,
how will they do that? Because I don't see any option to separate them here.

[LoyceV]

Syncing Headers is still 25% complete.

Syncing Headers usually takes only a few minutes. After that, you'll have to sync and verify the full blockchain (700+ GB), which takes much longer.

Will I be able to make Bitcoin transactions even if it is not fully completed?

You can receive transactions, but won't be able to see them yet.

2. If my PC ever crashes, if I have to reinstall Windows, and later install Bitcoin Core again, how will I get my previous wallet back?

The same way as any other data: make backups. If you lose your wallet, you lose your Bitcoin.

3. Suppose I have 5 members in my family. We only have one PC. So, if everyone in the family wants to use Bitcoin Core,
how will they do that? Because I don't see any option to separate them here.

If 5 people use the same user account on the same PC, you may not be looking at the best setup to securely store your Bitcoins. Is there a reason 5 people in the house all want to use Bitcoin Core, or is this only hypothetical?

[DarkState]

The same way as any other data: make backups. If you lose your wallet, you lose your Bitcoin.

So what is an efficient way to backup? Should I copy the files to a separate device after every transaction in my wallet?

If 5 people use the same user account on the same PC, you may not be looking at the best setup to securely store your Bitcoins. Is there a reason 5 people in the house all want to use Bitcoin Core, or is this only hypothetical?

There is no specific reason. I'm just wondering if there is an option to run two separate Bitcoin cores on a PC in case such a situation arises in the future.

[Cookdata]

I have some confusion about Bitcoin Core. I want to clear these up a bit.

1. I have Bitcoin Core version 30.0 installed on my PC. But Syncing Headers is still 25% complete.
Will I be able to make Bitcoin transactions even if it is not fully completed?

You can't spend bitcoin with bitcoin core if your are not fully synced, your node (Bitcoin core) has not finish validating previous blocks and utxo, you have to wait for the initial block download to reach the latest last block before you can send transaction.

You can receive though while waiting for your core to fully sync but you have to wait for it to finish sync before you will see your balance.

2. If my PC ever crashes, if I have to reinstall Windows, and later install Bitcoin Core again,
how will I get my previous wallet back? I haven't seen any authentication login option in Bitcoin Core.

 You can backup your wallet.dat just incase but the more you make transaction, make sure to backup your wallet.dat so your labels, private keys with new generated addresses and metadata will be backup

The last time I experienced window crashed, I removed my my from crashed laptop, inserted it to my external case and connect it with my mini laptop and then transfer everything, I have some other files that I don't want missing.
I loaded a new window and transferred back my file from the mini hdd to the main laptop.

This is the easiest way.

3. Suppose I have 5 members in my family. We only have one PC. So, if everyone in the family wants to use Bitcoin Core,
how will they do that? Because I don't see any option to separate them here.

You can only run one Bitcoin core with one PC, you may run Bitcoin knot but you have to disable one of them, the two app can't work at a time because they use the same bitcoin folder.

You can create new wallet if you want with one laptop but it's going to be power by one Bitcoin core.

[LoyceV]

So what is an efficient way to backup? Should I copy the files to a separate device after every transaction in my wallet?

USB-sticks, external hard drives, CDR, DVDR, you name it. Considering the level of questions you're asking, you're probably better off reading a bit more about Bitcoin Core first. Just search, this information can be found in many topics.
Remember: "one backup is no backup"!

There is no specific reason. I'm just wondering if there is an option to run two separate Bitcoin cores on a PC in case such a situation arises in the future.

You can, on different ports, but I wouldn't recommend it. Electrum is much easier if you want multiple user wallets.

[Cricktor]

Immediately you create your wallet, backup your seed phrase and save it somewhere safe where only you can access anytime anyday.

You can also backup your wallet.dat just incase but the more you make transaction, make sure to backup your wallet.dat so your labels, private keys with new generated addresses and metadata will be backup

Are you sure you've ever used Bitcoin Core? Because Core doesn't use mnemonic recovery words to represent a random initial entropy to derive a HD wallet.

Recent descriptor HD wallets generated by Core have a few master key descriptors which could be backed up and would make it possible to recreate the wallet from such a backup. But this requires a bit of knowledge and isn't particularly straight forward.

Backing up the wallet.dat file, preferably from the File > Backup Wallet... menue, is still the way to do a proper Core wallet backup. With a HD Core wallet one intact backup wallet.dat is sufficient to update the wallet at any time because all keys are deterministic.
You should always have multiple redundant wallet.dat backups to avoid ever loosing your wallet completely.

[Cookdata]

Immediately you create your wallet, backup your seed phrase and save it somewhere safe where only you can access anytime anyday.

You can also backup your wallet.dat just incase but the more you make transaction, make sure to backup your wallet.dat so your labels, private keys with new generated addresses and metadata will be backup

Are you sure you've ever used Bitcoin Core? Because Core doesn't use mnemonic recovery words to represent a random initial entropy to derive a HD wallet.

I messed up things from the beginning when forum was misbehaving with cloudfare and drafting my reply.

[nc50lc]

If 5 people use the same user account on the same PC, you may not be looking at the best setup to securely store your Bitcoins. Is there a reason 5 people in the house all want to use Bitcoin Core, or is this only hypothetical?

There is no specific reason. I'm just wondering if there is an option to run two separate Bitcoin cores on a PC in case such a situation arises in the future.

If you really have to separate your wallet, you may create a shortcut containing a command line arg --wallet="path_to_your_custom_wallet_location" containing your "your_wallet_name" folder that has your wallet.dat file.
Of course, unload your wallet first, close Bitcoin Core, and move "your_wallet_name" folder from .bitcoin/wallets to your preferred personal/hidden directory. (that'll be used in the shortcut)
Then save that shortcut to your personal folder.
(instructions depend on your OS)

With that set-up, if you use the shortcut to start Bitcoin Core, your wallet will automatically be loaded on startup. (including their wallets)
But if your family members use the normal shortcut without the command line arg, it'll only open their loaded wallets.

But be warned that this essentially loads and unloads the wallets as you open and exit Core using the shortcut.
So in a "pruned" setup, it may get to the point of being unbale to rescan if you left it unloaded for a few weeks (depending on your prune size settings)

On the other hand, separating instances of Bitcoin Core will require you to spare separate bitcoin data directories which will take a lot of disk space.
You may use pruning but that will come with limitations. (DYOR for it)

[ABCbits]

There is no specific reason. I'm just wondering if there is an option to run two separate Bitcoin cores on a PC in case such a situation arises in the future.

You can, on different ports, but I wouldn't recommend it. Electrum is much easier if you want multiple user wallets.

Bitcoin Core create lock file, so multiple Bitcoin Core app can't use same folder path to reduce storage usage. Electrum encryption also means other people can't even see your balance, address and TX history, which also is better in terms of privacy.

[Floczy]

If 5 people use the same user account on the same PC, you may not be looking at the best setup to securely store your Bitcoins. Is there a reason 5 people in the house all want to use Bitcoin Core, or is this only hypothetical?

Permit me to come in here, Loycev. I was thinking 5 people can actually share the device with only one Bitcoin core application installed. This is my idea;

5 people, 5 external hard drive, each having their own wallet.

1. Each person should plug in their personal external drive when they want to access Bitcoin core app.
2. Launch Bitcoin core
3. Load their wallet from the drive
4. After they are done they should Bitcoin core and unplug their drive.

Dont you think this could work, let me know your feedback, im learning. Because I think since each wallet is separate in the external drive, they wont have problem of security.

[nc50lc]

-snip-
Dont you think this could work, let me know your feedback, im learning.

Let me: it will work.
This requires to use loadwallet command provided with the absolute path to the wallet and without persistence flag,
Because Bitcoin Core's (GUI) Open Wallet menu wont see the wallet in the external drive when a user wants to load it,
while using "Restore" will create a copy of it to the datadir which isn't what the users want.
So, it has to be loaded via command.

Alternatively, they can use the shortcut method that I've share above but the path to each external drives shouldn't change upon plug/unplug for the shortcut to be reliable.
(e.g.: OS like Windows assign letters that may change depending on its availability)

Note: If they missed to unload the wallet before unplugging the drive, the saved wallet path in their Settings.json file will break and Bitcoin Core will start with warning in every start.

Because I think since each wallet is separate in the external drive, they wont have problem of security.

Not safe at all,
Even if the wallets are in separate external drives, using the same machine is still unsecure
since your private keys will still be saved in that machine's RAM for a short while when the wallet needs them (when signing a transaction, etc.)
So your safety is so thin that getting hacked is just a matter of having one bad actor among those 5 users.

[LoyceV]

5 people, 5 external hard drive, each having their own wallet.

I assume this is also only hypothetical, which kinda makes it pointless. Of course it can work, but it's not very practical to do. It's a hassle, and easy to mess something up.
You can just as well install enough disk space in one system, and have each user download their own blockchain in their own (encrypted) home directory. Or just use Electrum, it even makes more sense to create your own Electrum server for those 5 users, than have all of them share a Bitcoin Core installation.

[ABCbits]

5 people, 5 external hard drive, each having their own wallet.

You could just allocate the money to buy 5 hardware wallet, rather than 5 external hard drive. It's both more convenient and secure (assuming you buy hardware wallet from reputable company). Black friday is still ongoing when i write this post, see https://bitcointalk.org/index.php?topic=5423037.40.

Because I think since each wallet is separate in the external drive, they wont have problem of security.

Not safe at all,
Even if the wallets are in separate external drives, using the same machine is still unsecure
since your private keys will still be saved in that machine's RAM for a short while when the wallet needs them (when signing a transaction, etc.)
So your safety is so thin that getting hacked is just a matter of having one bad actor among those 5 users.

And let's not forget about more basic security issue, such as keylogger.

[satscraper]

3. Suppose I have 5 members in my family. We only have one PC. So, if everyone in the family wants to use Bitcoin Core,
how will they do that? Because I don't see any option to separate them here.

I think in this case the easiest yet still reliable option is to use Bitcoin Core as the node rather than as wallet, and then connect the client to it that supports multiple wallets at once. I would recommend Sparrow. This client can connect directly to Bitcoin Core, allowing each of your family members to create the separate wallet for themselves and enjoy full privacy and security for their funds. ^{All wallets may be managed by Sparrow at once or separately}.