That's a very open ended question.
Basically using C++, the Windows SDK and DirectX 12 as well as a couple of libraries doing some of the work (such as Bullet for physics, OpenAL Soft for audio and both the Oculus SDK and OpenVR for the different VR headsets). There is also some code for Linux and macOS in place as well as some rendering with Vulkan and Metal.
I could write a lot about almost everything involved (window and swap chain creation, render pipeline in general, input, model loading, audio, shadows, multisampling, physics, maths, ...). There actually are some articles on a very outdated website about the engine: http://rayne3d.com/blog many things have changed since then, but some of it still works essentially the same. I also started writing devlogs for my next project on my website (only two so far, as I just got started): http://slindev.com
If you have a more specific question, that would make it easier to answer :)