It's Reddit but federated. A federated service is one that works like e-mail, i.e. there are multiple providers/servers/instances but all of them are connected so it doesn't matter which one you choose. Additionally, Lemmy is federated to other services (e.g. Mastodon), forming what's known as the Fediverse.
So one thing I don't understand is, how do all the federated servers find each other? Is there one server that maintains the registrations to all other servers and each server can pull from there? What happens if that central server decides to delete another server from it's list, doesn't that put authority back into one server/persons hands? Or does each server maintain their own list of federated servers and if so you never know if you're fully connected / how much time would that take each server owner?
No, there's no central server. To my knowledge, servers federate either manually or by their users manually exploring other servers. But most servers at the moment are already federated with each other.
What's the difference between the multiple lemmy instances and something else also ActivityPub based like kbin? Is lemmy.ml vs lemmy.world the same as comparing lemmy.world with kbin.social? I have accounts with both Lemmy and Kbin and confused a bit by what I'm seeing, and also Threads vs Magazines vs Microblogs haha. So much to learn about the fediverse!
Well, the software they run is different. The frontend looks different, etc. Lemmy can connect with kbin, as well as Mastodon, so it's just one big network at the end of the day.