For a community to be visible, any 1 member of the instance must have searched about the community on that instance (after which it'll get stored in local db and become visible to everyone else).
No one must have searched c/youtube of lemmy.ml here and vice versa.
I'd still recommend you make your own c/youtube if you're interested though. Many communities on lemmy.ml were made earlier and have inactive mods so this could become the primary and main youtube community.