They’re what is called federated. They use the same protocols which allow each to see the other. The best comparison I’ve heard to describe it is like how you can read emails irrespective if someone used Google Mail, Yahoo Mail, or Microsoft Outlook.

Do you mean that their API’s are similar or the same? That’s why they can interconnect?

Yes and no. It's not just the protocol but also the fact that Lemmy and bin both are fairly similar in function, making interoperability / connectivity much easier. kbin is probably even a little closer to Reddit in its native desktop layout than Lemmy, which most Lemmy apps also are following. It's one of the reasons why I went with kbin instead, but I also have a beehaw account which I primarily use for mobile testing, as kbins API isn't released yet, which means there's a distinct lack of proper mobile apps at the moment. Here's how my regular frontpage looks like at the moment, with a couple userscripts.