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, because Beehaw doesn’t accept them, and therefore doesn’t pass them along to the other instances, either.