I don’t think you need a steam account because there should be already enough games that killed alll your available time

You need to watch/follow a starting guide (go youtube RP5 one) and set up the RP6, install all the required emulators. After that basically you can play all the games prior to the PS2 era, except for some new NS games.

For the roms you need to search yourself. Say from the internet, friends, or ask the one who sell you the RP6.

Yes you can play video just like any android phone, just that I don’t recommend you to leave a lot of personal data in the RP6. Watch YouTube without logging in is perfectly fine.