Or is it only the 3 that you can pick from where you can create an account in settings?

Still new so trying to work this out.

Is Connect for Lemmy only federated with a few instances?

For example I wanted to join lemdro.id As it’s the replacement for /r/Android But when I search it’s seemingly not there.

From what I can tell that is the Instance, and within it are communities. Are we able to add additional instances? Or even post across them? (I think I’m on lemmy.world?)