there are lots of games like www.voxiom.io, www.diep.io or www.agar.io where you play multiplayer games within the browser and they are sometimes even built in a 3d environment.
How do they actually build these games ? Is it really based on html/css ?
Yep, it’s based on html/css/javascript usually. With things like WebGL you have a full 3D rendering engine in the browser. They’ll often use WebRTC or web sockets for game updates from the server/other players.
There’s a list of HTML game engines if you want to dig deeper: https://html5gameengine.com/