> I do know that if you create a single account it is trivial to join any "server"

Only if it's public. There are many private Discord servers.

The way they do it is that the default set of permissions is basically none, but then there's a server role which actually gives you permissions to see the channels and post in them. So, anyone can join the server, but only people who have been granted this role (e.g. by admin) can do anything on it, or even see others.