I don’t know if this is in the App Store version, but if you put in community@instance it’ll pull up communities even if your instance doesn’t know about it yet (this might not be in the App Store version, it’s new).
If that isn’t there yet, if your instance doesn’t know about it yet you still have to go to the web to force federation.
If your instance knows about the community, the search tab will find it and there’s a subscribe button there.
This must be one of the fixes the dev team made between App Store submission and approval because my TestFlight version has the links available and opens YouTube. This fix will likely be in the next App Store push, I think Gavin said tonight or tomorrow so hopefully publishing will be soon.