To be clear, the Fediverse doesn't mean that everything is interconnected. It means that everything can be interconnected, but most sites will only do a very minimal form of interconnectivity. And that's mainly due to personal choice. You wouldn't want to have Instagram posts on your Reddit feed, and you wouldn't want Tumblr posts on YouTube. You can do that, but why would you?

So most sites will only interconnect with other sites that they deem to be similar enough in content style. Lemmy interconnects with Kbin because both are Reddit clones. Kbin interconnects with Lemmy, but it also interconnects with Mastodon. Apparently the developer of Kbin thought that Mastodon is similar enough in content style that people would appreciate having Mastodon posts appear on Kbin. And this happens for all the other sites. The Fediverse is less like a tightly connected network, and more like a loose connection of sites that could operate together, if they ever chose to do so. Like a federation, if you will

Basically, if you're on Lemmy (which you are), you're only going to see Reddit-like content