The doom source code was released. If you would like a guided tour, maybe look at https://fabiensanglard.net/gebbdoom/ https://fabiensanglard.net/gebbdoom/ Unr

The doom source code was released. If you would like a guided tour, maybe look at https://fabiensanglard.net/gebbdoom/ https://fabiensanglard.net/gebbdoom/ Unreal is source available too, if game engines are of interest to you