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

Even with federation, [email protected] would be a completely different community from [email protected]. (like email: [email protected] is a completely different account to [email protected])

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 [email protected] from within your gmail webmail)