If you’re familiar with IRC it works almost exactly like that but via a website. Your browser is your client and you can connect to and sign up accounts on any server. Channels can be seen and interacted with across instances though but they are still seperate entities, which is the only difference.

Although it seems like there would be multiple communities and fragmentation what usually ends up happening is that your server fedeates other popular subs though others interacting them and they still gain traction and get popular.