Probably not the only one, but there’s not very many of you, because there are plenty of engines. Unreal, Unity (rip), and Godot are the big general-purpose ones, for good reason: they’re modern, powerful, and most importantly easy to work with. But there’s also Source, CryEngine, Frostbite, id Tech, and many more: https://en.wikipedia.org/wiki/List_of_game_engines
No, the demons are the system processes: https://www.cs.unm.edu/~dlchao/flake/doom/chi/chi.html
Which I guess makes the player the DRM.
What is old is new again
The dev studio name is Reburn, since they buried it all the way down in the fourth paragraph.