Doom is famous for running on everything from graphing calculators to ATMs, so I guess it shouldn’t be too surprising to see it playable here inside another video game, especially since that game is Minecraft.
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: