So would “communityA” on beehaw have different content to “CommunityA” on Lemmy.world due to the defederation/(block)

Even with federation, communityA@beehaw.org would be a completely different community from CommunityA@lemmy.world. (like email: mazkarth@gmail.com is a completely different account to mazkarth@hotmail.com)

What federation does for you is that you, on lemmy.world, can access them both with a single login. (email: you can write a mail to mazkarth@hotmail.com from within your gmail webmail)

If you host your own, do you need to establish federation with all other instances or only with the ones you want to use communities from?

If I only federate with lemmy.world, would I be able to see comments on /c/selfhosted@lemmy.world on my instance made by a user from lemmy.ml?

Would a user that reads /c/selfhosted@lemmy.world on lemmy.ml see my comments, if I only federate with lemmy.world?