Out of the box it can play audio in the background, and now that extensions are available you can block ads as well. You don’t really need constantly updating 3rd party clients or questionable firewalls anymore if you just want usable youtube on a phone.
Are youtube ads in Firefox on iOS blocked? They are with uBlock on Firefox Android, but iirc Firefox on iOS doesn’t support extensions.
IIRC AdGuard iOS content filter works on safari a while ago to block youtube ads, but that was before youtube began its war against adblockers. Haven’t tested it again yet.