Well they all use a different interface style. For example Lemmy is "Reddit like". Mastodon is "Twitter" like. So just use whatever platform works best for you. You're on Lemmy right now so I would guess you're looking for a Reddit style. There's also kbin to serve the same, but they're having some problems federating right now. They may not offer the best user experience. kbin is newer and still in early development. Lemmy has a few years under its belt.
So is kbin part of lemmy? Or is it different like mastadon?
Kbin is another Reddit like platform. All Fediverse platforms communicate using the ActivityPub protocol, but each platform has it's own development team. Right now ActivityPub is not seamless between platforms, for example Lemmy is not talking to Mastodon very well right now. The goal is for seamless communications between platforms, but that's not the case yet. It's just growing pains that should get worked out over time.
May be a controversial opinion but I don't think lemmy instances should talk to mastodon, I feel like twitter and reddit are two entirely separate platforms for entirely separate purposes and if you mix them together you kinda destroy that individualisation
We were about to read Kbin posts from Lemmy.ml just a week ago but I think the server updates might have caused federation issues cause I no longer see them on my feed.
But yes, you should be able to subscribe to Kbin "magazines" (their version of communities/subreddits) and see their posts from Lemmy and theoretically vice versa.
Different, just providing more or less a similar experience making it more compatible to lemmy by nature, technical difficulties still exist tho, like it was said before, obviously, but that's normal, things are still in development