Let’s say you’re on your lemmy.ml account, and you get banned from the lemmy.dbzer0.com instance, you would just be banned from that instance not all instances. But if you got banned on lemmy.ml from lemmy.ml then you’d lose access to that account. Alternatively, you can also get banned on a community level as well. In regard to your accounts with the same name between instances there is no link, they are entirely separate.