This “You Should Know” post is top of ~June, for good reason. This post announced the kbin.social Federation/compatibility with Lemmy.
A huge amount of test-posts from a wide-variety of instances came in to test the new federation abilities (including from kbin.social, one of the biggest kbin communities).
I think this post shows the hope that over the long-term, we can get federation across the larger fediverse. I don’t know if Mastodon is in the works, but these kinds of posts give hope.
Sure, in that case you can interact on any beehaw post with beehaw users and any users from instances federated with beehaw, or interact on any lemmy.world post with lemmy.world users and any uses from instances federated with lemmy world.
But if a post is created on beehaw and a lemmy.world user subscribes to it, lemmy.world downloads a copy of the post for lemmy.world to interact with which is separate from the original post on beehaw. So you would effectively have access to 2 different versions of the same post, one with comments from beehaw users & federated instance users, and another with comments from only lemmy.world users.
In practice you’d only ever really see the first version of the post though, same as if your account was on beehaw in this example. There’s no solution where you could interact with both beehaw and lemmy.world users on the same post from being based on a third instance that’s federated with both.