It’s just like with email, you choose which provider you go with. You must have gone to lemmy.world when you first came to Lemmy, so that’s your current provider.

Right now, you can easily just go and sign up with another instance if you like. There are a heap of them, and if you google you can find out more about different ones. Some are general, like Lemmy.world, some have a geographical focus, like aussie.zone. Others are based more on shared interests, like ttrpg.network.

On Lemmy, the equivalent of Reddit’s "subreddits" are called "communities". And each instance has its own communities. Right now we’re commenting on !showerthoughts@lemmy.world, which you might also see referred to as /c/showerthoughts@lemmy.world. Those both mean the same thing. But there’s also a !showerthoughts@sh.itjust.works, and those two are completely separate communities with their own mods and their own rules. And even though your account is on lemmy.world, you could go into that other community and post or comment.

Each instance has its own admins as well, and those admins have powers similar to that of admins on Reddit. They can ban users, remove communities on their own instance, etc. Most often the thing you’ll see talked about in terms of admin powers is "defederation". That’s where the admins of one instance prevent users from another instance from interacting with them. Different admins might have different policies for why they would or would not defederate another instance, and you may want to ensure the instance you use has a defederation policy that you’re comfortable with—and one where your own instance hasn’t been defederated by other instances that you want to interact with.

Right now, to move to another instance you just have to create a new account. Your old comments and posts etc. will be left behind. But there has been some talk about being able to migrate your account from one instance to another, and it’s theoretically possible. Mastodon is a fediverse equivalent to Twitter (in the same way that Lemmy is a fediverse Reddit) and I believe account migration is possible on Mastodon. For the most part though, no, you won’t really notice the difference between different instances.