You may find confusing how there could be at the same communities (subreddits) with the same name in different instances. You can use the Lemmy Community Browser to find communities easily among different instances.
You're posts are visible from other Fediverse sites, like Mastodon or Kbin and you can interact with their users.
An user or an instance can block users or instances. If you have an account on lemmy.world and the instance admins blocks another instance you won't be able to see the posts or comments made from their users.
Is there anything in particular that you want to know?
Not the most active! The most fitting to your needs. If you dont like the instance, then dont go there! Search for the most comforting Community and MAKE it active by contributing to it!
I mean, at the moment I'll just take most active. I believe in the idea of the fediverse giving us multiple communities for different vibes of talking about the same topics but for now I'll just take having communities that are a bit more active
Are you familiar with the fediverse ? You can interact with people and communities on different instance the same way as you can sent an e-mail from hotmail to gmail.
Instances are managed by individual and non profit. Just-like big social media, they can decide to turn-off an instance (remember google plus ? ) for whatever reason, and your MPs are on that person server (again the same as big social-media, that's even their business model). A coll thing is that if you have 5 shady person and 95 honest person it's still better than a big social media which is 5% dishonest with everybody (and I'm sure you've heard enough about threads and tik-tok being spyware to know that big-social media are more than just 5% dishonest), but use common sense regarding cyber-security (don't use Lemmy's MP for sensitive informations)
Even though the project has been existing for a few years, we're all pretty new here, so we don't know much more than you.
Quickest way to get you up and running is to find the equivalents communities on here to the ones you liked on Reddit (assuming you are a Reddit refugee) and sub.rehab is probably the simplest to use. Don't worry if there are duplicates, subscribe to them all.