Asklemmy

You can choose whether or not your instance is federated - you can also choose if you want to only allow specific instances or block specific instances. Once you start interacting, for example subscribing to other instances, the number of servers federated with your server will build up.

Here's a bit more about how that works: https://join-lemmy.org/docs/en/administration/federation_getting_started.html

By the way, this question would be better off in [email protected]

Depends. I don't know much about the ActivityPub standard yet, but on my matrix instance I could set it up without federation, or with federation and a blacklist (blocked nodes), or with federation and a whitelist (allowed nodes).

So it depends on your setup, and the setup of other instances.

You can pretty much just start adding other instances. Most of them will federate with all comers, and manually ban any instances from federation that cause trouble.

And you don't exactly add the instances. Someone on one server subscribes to a community on the other, and the instances talk to each other to make that happen.

No need to be approved. Unless, of course, you do something and get blocked by some instance. There also are some instances that block everyone - I think that's the Private Instance setting in the setup UI.

Yes, once you have a server and domain you can read the lemmy docs and set up an instance of your on that server at that domain. You can then make an account on your own instance, and from that account do all your subscribing and posting and commenting on all other instances which haven't defederated from your instance, which at the start will be none, and will probably remain so unless you start doing something unsavory. You can open your instance up for others to register or not. Regardless all communities created on your instance will also be available to every other instance (again, barring defederation).

selfhosted should have more helpful answers

Most of the servers running lemmy have federation enabled and open (as you can see, I'm one of those self-hosted instances) and you can just interact with them. Some do have federation locked down, but that's the minority