My understanding is that Lemmy isn’t peer-to-peer, it’s more like hub-and-spoke, relative to the instance that the community is on.

So if users on B and C are interacting with a post on A, then it’s the responsibility of A to be the postman and handle the syncing between A<>B and A<>C; B and C don’t directly talk in this case.

Thus, nobody outside of Lemmyworld sees posts from Lemmyworld made on Beehaw communities, because Beehaw doesn’t accept them, and therefore doesn’t pass them along to the other instances, either.