I'm new to it as well and I've been poking around for info recently, so take this with a grain of salt.

Fediverse = Lemmies + Mastodons + Pixfed + all the other things that use Activity pub. They can, more or less, all talk to each other. So you can, more or less, get stuff from Lemmy on Mastodon and vice versa. But for right now, it looks like that can be a bit of a hassle -- or at least more of a hassle than we're used to with centralized services like FB, IG, Twitter, etc. You generally have to look them up and get a specific link and search on your own instance and follow.

But the fact that they can do this at all, and that so many people are working on making it better, means it will probably become easier and easier over time. And the more you play around with them, the less of a hassle it becomes.

My takeaway, for the moment, is that it's simpler to keep Mastodon and Lemmy as two separate things, but it's not so hard to cross instances within each of them. And since I am still new to this, I've joined a few instances of each to get a feel for them. Signups for both are really simple. The main thing I look at to decide if I'd want to join is their local feeds -- the things you could browse easily if you joined that instance. I feel like it gives you a sense of who's there and what they're posting and what the instance's main interests are.

I've been using lemmy.world as a general instance and joined beehaw.org and mander.xyz for more focused instances with different approaches to moderation and federation. But following communities across instances is pretty easy to do once you try it out. So that means you could have just one single instance and still follow interesting people or communities across all the other instances out there.