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.

Trying to figure out how to add !tafkars@feddit.de checked a couple of lemmy instances and it doesn't seem to exist. Though on lemmy.world I am able to view. I have tried adding it to search a couple times but it doesn't seem to find/do anything. Is it just that feddit.de or fmhy.ml are not able to talk to each other?