Do I need a Coinbase Wallet, as well as the Coinbase account?

[hosseinimr93]

that machine MUST go momentarily online for that creation process.  Correct?

No.
It's not that the seed phrase is generated by a third party and is given to you.

Any seed phrase is only a representative of a large random number.
For generating the seed phrase from that random number (which can be generated by your operating system), you need to use a tool like electrum. You can even do it yourself, if you have enough mathematical skills.
Electrum doesn't do anything more than this when creating a wallet and doesn't have to be online.
All the process of generating the seed phrase can be done offline and developers don't have any control over your seed phrase, your wallet file, your transactions, etc.

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[NeuroticFish]

NeuroticFish, thanks for that link.  
That explains the process almost as clearly as You just did, but still...  
When ORIGINATING / CREATING a non-custodial wallet (for the intended Offline machine),
that machine MUST go momentarily online for that creation process.  Correct?
Or am I still missing something?

Aahhh...  I just read the reply from Jerome...
With Electrum, I can create my own 12-word-seed-phrase (or any other text?).
Now, THERE's the answer for paranoids like me.
Nobody except ME will initially see that source seed-phrase.

Am I clear, now?    

At creation, a new random seed is generated. Basically a random set* of 12 words from this list is picked: https://github.com/bitcoin/bips/blob/master/bip-0039/english.txt
It's not your own list of words, but you can add custom words though to it if you want.
It's ... random. For generating random numbers (or lists) a computer doesn't need to go online.


* actually a checksum is also made (and Electrum adds its own version info too) so at least the last word may not be exactly random, still the explanation can be seen as a simplification, not too far from reality.

[hosseinimr93]

* actually a checksum is also made (and Electrum adds its own version info too) so at least the last word may not be exactly random, still the explanation can be seen as a simplification, not too far from reality.

In an electrum's seed phrase, all words (all 132 bits) are generated randomly.
Electrum's seed doesn't have a checksum same as a BIP39 seed in which the last 4 bits are checksum.
Electrum only checks the first 8 bits of the hash of the seed phrase to see if it can be a valid seed phrase.

[Mark228]

Crap.  I looked at Electrum, and it deals only with BTC.
I need ETH, 'cause I plan to sell NFTs on Opensea, earning ETH.
Crap.

Regardless, this is quite Interesting and Educational.  I'm in.
Re: Electrum, I'd download the Installer to my Online machine, copy it to my Offline machine, then run it offline.
That Installer includes the 2048 words to choose from.

May I instead create my own seed-phrase, like
ThisIsMySecretPhraseThatNobodyButMeKnowsBecauseIMadeItUpAllByMyself
Are there Any ETH wallets that would allow me to do that?

[hosseinimr93]

I need ETH, 'cause I plan to sell NFTs on Opensea, earning ETH.

For ethereum, you need to use another wallet.
The best is to buy a hardware wallet like Trezor. It's open source and can provide the maximum security.

There are also many multi-currencies wallets. But the problem is they are close source.

May I instead create my own seed-phrase, like
ThisIsMySecretPhraseThatNobodyButMeKnowsBecauseIMadeItUpAllByMyself

Technically, it's possible to force electrum to do so. But since that's not a standard seed phrase and you won't be able to generate a same wallet using other wallets, that's not recommended at all.
If you want to have a valid seed phrase with your selected words, you must select 11 words from BIP39 wordlist and brute-force the 12th word.
Of course, it's not a recommended method and you should let the seed phrase to be generated by electrum. Humans are not good at generating random data.

Are there Any ETH wallets that would allow me to do that?

For ethereum I don't think there's any.
Again, it's not recommended to have a seed phrase with your selected words, even if it's possible.

[Mark228]

Right, Hoss, cannot use Electrum, because I need ETH.
OK, I'll Trust that...  wherever I get a wallet from, They won't save the seed-phrase.
My paranoia is probably overkill.

Do not yet want to spend $ for the Hardware wallet. 
Hopefully, I'll soon get Big$, and Then I'll get one of those.

I just read about the open-source Exodus wallet.
ONLY on a desktop (not mobile), but Desktop is what I want anyway.
Any reason to Not go with Exodus?

[hosseinimr93]

I just read about the open-source Exodus wallet.
ONLY on a desktop (not mobile), but Desktop is what I want anyway.
Any reason to Not go with Exodus?

As far as I know, exodus isn't open source. Both desktop and mobile versions are close source.
I have used exodus before (only for holding some altcoins in short term) and honestly never experienced any problem. Also, I haven't ever heard of any security issue with exodus till now. But since it's close source, I still wouldn't trust it, especially for long-term holding.

For ethereum, you can also try metamask.