All of that is extremely possible with a web app though, and most of it is probably done even easier with a web app. In fact that is the usual way to build a we

All of that is extremely possible with a web app though, and most of it is probably done even easier with a web app. In fact that is the usual way to build a web app, make a server that does the heavy lifting and then make a GUI that runs in the client which can then be accessed by multiple users. You can easily access them from a smartphone too. The only exception is this: "Another case might be several users running on one machine each with their own displays and inputs." This would be multi-seat which doesn't really have anything to do with the display server. It is implemented in udev and logind, which spawns additional X or Wayland servers for each additional "seat".