1. What is a good offline wallet I could use to store BTC? Are download btc wallet or app would be reliable?
Recommended wallets- Bitcoin Core: download it from Bitcoin.org or Bitcoincore.org. Bitcoin Core is too heavy with full node whilst with prune node, you can have other technical problems. I won't recommend you to use Bitcoin Core with prune node, so if your computer does not have too much space (about 500 GB), let's consider Electrum wallet that is lighter
- Electrum wallet. It has an application for Android and please get Android link to download in the website page, don't search Electrum app in Google Play
- Wasabi wallet. It gives you Tor connection, Coin Join transaction and better privacy
- How to Safely Download and Verify Electrum. See the guides to verify Bitcoin Core or Wasabi in download pages
2. How many Keys and what are those keys that I need o save from the BTC wallet?
Depends on the wallet you are using. You can save the mnemonic seed of whole wallet and use such seed to recover all keys, addresses in that wallet. Or you can simply save the private key of a specific public address you store your Bitcoin.
3. If I bought BTC and not successful. Is there any refund I can get?
If you are Bitcoin buyer, it depends on how your trade was done. If the trade is done without Escrow, you won't get anything. Scammer will get your fiat and run away.
If you are Bitcoin seller, remember that Bitcoin transaction is irreversible so make sure you receive fiat (or other payment method) before you do your last step "Release your Bitcoin" for that trade.
4. Can I transfer BTC to someone living in different country?
Yes. You can. Bitcoin transaction and its network are Borderless.
5. What are the Tips in handling or having BTC in the wallet for safety keeping?
- Write down private keys or mnemonic seeds that help you to recover your wallet later (on same or different devices)
- Save backup files in secret and safe places. Should be Offline.
- Don't make noise and tell everyone that you are a Bitcoin investors
- Try to practice wallet recovery, and do it successfully before you buy your first bitcoin and store it in that wallet