Hit the communities button near the top of the page to subscribe and view. You don't need to sign up to more instances unless you really want to, we can post on any federated instance. It's weird at first but you'll get it.

Discoverability is something that could definitely use more work. Right now I recommend the site lemmyverse.net/communities, which is searchable and shows subscriber and active user counts. It should help you find where the most populated communities for your interests are located, if they already exist over here.

Your front page has three feeds. All is just what you'd expect. Local filters to only show posts from your home instance (lemmy.world in your case). I find it's mostly useful if you're registered to a smaller instance and want to keep up with local concerns.

Home shows updates for all communities you are subscribed to.

I'm on Lemmy.world as I assume we all are

We're not! :)

I'm on sh.itjust.works, others might be on lemm.ee, lemmy.zip, feddit.uk, etc., but we can all interact with each other. That's one of the benefits of the fediverse.