From Lemmy you can’t use Mastodon (though you can the other way around - from Mastodon to Lemmy, but it’s suboptimal). Matrix follows similar ideas as Fediverse, but isn’t part of it, there’s no way of interacting with it. Kbin is similar to Lemmy and you can actually interact with it - visit for example !AskKbin and interact with the community. You won’t notice any difference, but you’re interacting with Kbin community (on Kbin you’ll find the community on kbin.social/m/AskKbin).