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)