Lemmy has been around longer, so it is probably more robust, but Kbin has more features, including the ability to follow individual users across the 'verse, and has support for a number of Fedi groups, including Friendica Forums and Guppe Groups, that Lemmy has no support for.
It is its own community that happens to be federated with Lemmy and Mastodon. You can toggle federation on and off, and when off, you only see kbin's material. Otherwise it functionally acts like another lemmy instance, with a different interface and features missing from base lemmy.
There is also a microblog attached to it that connects with Mastodon in the same way. Each community has its own microblog that acts as a hashtag when on Mastodon.
And Lemmy, they all talk to each other via ActivityPub. There's a number of commenters in this thread that are commenting from Mastodon and Kbin (like yourself) while I'm on Lemmy.
They are different platforms, you can access Lemmy communities from Kbin, where it looks just like a kbin magazine, or from Friendica, where it looks just like a Friendica Forum, of from Mastodon, where it looks like an annoying bot that re-posts every comment as a separate Re-toot. ---
There are many other Fediverse platforms but those are the ones I have the most experience with.