Most people use the email analogy; the sub-lemmys are on different servers owned and moderated by different people, they just happen to use the same communication protocol. Similar to how you can email a hotmail, yahoo, or icloud account from a gmail account. And because they’re different servers people might have the same name, just differentiated by the server name. So you might see a MarySmith@gmail.com and a MarySmith@hotmail.com who aren’t the same person, they just happen to share a name.

It’s just like email (or Usenet on the chance you’re old) - you have an account on lemmy.world, you can only log in on lemmy.world, but you can read and reply to users and posts on fedia.io, mander.xyz, feddit.org, etc., et al

Ah, the domains / instances are simple:

Imagine that they’re houses. Your home is lemmy.world, my home is mander.xyz. We’re both visiting lemmy.blahaj.zone, and chatting together.

And just like your house can have a kitchen that’s completely separated from my house’s kitchen, two instances can have two separated versions of the same community.

With instances (domains) the content stays there until a user from another instance looks in on it, at that point the content is federated over on to their instance and it starts to spread…

At least that’s how I understand it, but I don’t really know all the technicalities to be honest

Don’t think too hard about it. Join the ones that seem active, ignore the ones that seem dead. That’s really all there is to it.

Anyone can run a copy (instance) of the software on a server and effectively create their own platform. These would be too small so the software allows sharing content with others (federating) if both consent.

It’s like lots of reddits, each can have the same subs. You can even spin up your own clone!