Fedilab - help me understand it - please

I downloaded fedilab from fdroid and it looks on the surface as though it can access instances of the fediverse as well as lots of other stuff - like I can see lemmy.world and lemmy.ml but not lemmy.ee but I'm not sure how to see individual communities - unless there's a post I can see to follow.

So my basic understanding of all this - coming from reddit, is that these instances all federate together - as one, so to speak.

I'm just getting a little confused here. So can anyone explain just how this piece of software works please?

Thanks.