The url for you would simply be lemmy.world. Just login with your account from the app and start scrolling, no need to migrate anything.

Federation in principle is actually really simple. Basically there are multiple servers (aka instances) run by different people and with their own urls, and they just send each other messages to stay in sync. E g. if you post something on LW, that server also sends it to all the others (all they are federated with), so they can show it to their users too. That’s it, that’s the magic.

The result is that all instances have the same content, and users can message each other no matter what instance they are on. So it doesn’t really matter which one you sign up on, and no content is lost if one of them goes down.