It’s not that simple and I don’t get why people pretend it is. The “instance” concept confuses everyone. The email comparison everyone uses is inadequate.
Edit: The issue is incredibly fundamental. You don’t just download the app, create an account, and go.
You download an app that lets you participate, then you need to decide your instance which is like deciding which specific browser you want tonuse that then lets you see individual communities. But also your instance might have a community with the exact same name as a community in another instance, so make sure you know the difference despite both of them being “news“ or “politics.“ Also all the different instances have different nuances to their federation and voting that you may or may not see, unless you are familiar with how each instance operates.