Most of those games are made with engines, for example, unity3d, unreal engine, godot… (c++, c#).
There is now a c# library for connecting to phoenix websocket, called Fenix.
You don’t need to write a new engine in Elixir, but You can use existing engine, and use Elixir as communication (match maker, presence, chat, lobby etc.)
I read some blogs about Elixir and Unity3D, and tried also, but it’s heavily under rewrite (Network, Entity system). It’s funny how they are trying to use multicore processing with there new ECS. It looks they have discovered benefit of immutability, and functional programming 
