super noob here. i have set up linux ubuntu on an old laptop. i have installed bitcoin core, verified, and DL'd the blockchain this past week (most up to date version). once the blockchain downloaded, i shut down btc core and the laptop and disconnected from the internet (for security?). then i restarted the laptop and btc core and created two wallets in btc core and encrypted both wallets and saved/protected the passcode.

I now want to back up these wallets so that i can make copies of the backups and not lose my coins. I have the following questions as i am very unsure how to know for certain if have everything properly backed up and saved.

1) by opening each wallet and selecting "backup wallet", will the file that is created be EVERYTHING i need in the event the laptop crashes or i have restore this wallet to a new computer?

2) I have actually created a backup file already, but the backup file that is created does not seem be a .dat file. when i save it to a USB stick or DVR and look at the file on a windows computer the file type says "21 file". is that correct or is the back up more than just the .dat which would be contained in the overall backup file?

3) is there a way, to confirm the back up i have made will actually work if it is actually needed? meaning, is there a way to test my backup?

4) my understanding is that if the BTC core wallet has "HD" in the bottom right, i don't need to dump the private keys, as the backup/wallet.dat file will have all that info. is this correct or do i need to make a physical backup of every private key as well.

thank you ahead of time. i really have searched for answers ahead of time, but i don't know what i don't know and not being able to recover things makes me extremely nervous.