Doom is famous for running on everything from .
It’s possible thanks to the VM Computers mod (via The Verge), which lets you build your own computer in the game using various parts, then use it as a portal to a virtual machine you’ve also got running at the same time.
In the above case, that virtual machine is playing Doom. But for a more meta example, here’s someone playing Minecraft in Minecraft.
For a quick explainer on how you build the PC and get everything up and running, this vid by Pandemic Playground is pretty good: