I've read that the Fediverse is a bunch of interconnected social media sites. Yet, when I try to log into a new Fediverse site using credentials from another, it doesn't work - I have to make a new account on the new site. For example, I made an account on Lemm.ee and assumed that I could use it to log into hexbear because they are both part of the Fediverse. This was not the case and I will have to make a new account for hexbear. This leads me to wonder though, how are the different Fediverse sites connected? They seem to be completely separate websites.
(oversimplified) they are "interconnected" just like emails are interconnected. I can log into Gmail and send an email to someone else at Hotmail. But I can't actually use Gmail credentials to log into Hotmail.
In the 'verse, "Federation" means every instance (I'm on aussie.zone) holds a copy of every other instance (I can see your post without ever visiting lemm.ee). This reply I'm posting is held in my instance, and a copy is sent to yours (and the whole Federation will also hold a copy and be able to see my comment).
If lemm.ee shuts down tomorrow, this conversation we're having will remain to exist in the thousands of other instances that are federated with lemm.ee today.