Think of it like an email address. Your Lemmy "instance" selection would kind of be similar to choosing to create an account on GMail. Even if your account is on GMail, you can still send mails to people on Outlook, icloud, and anything else, no matter which server you chose. Lemmy works similarly in a way: No matter where you created your account, you can interact with all instances. So I have an account on lemmy.world, but I'm commenting on your comment (you're on lemmy.one), while our conversation takes place on lemmy.ml. And it should just work