I have a question,
who knows how to do that? So that it shows such an info gui on the server on the page.
This is a scoreboard. You can make this appear with the Ingame MC command or you can program the plugin with Java as on GrieferGames.
How do I program it?
Using spigot. If you've never programmed, it's too hard to start with!
It's best to find a developer or download a plugin from SpigotMC.
Install a scoreboard plugin on a Spigot server. This is the easiest way to make an info scoreboard.