The N64 has a GPU, which is not the same as modern GPUs, and you need that GPU to render graphics.

Skimming the Github repo, looks like this uses rt64 under the hood for the raytracing support ( https://github.com/rt64/rt64 ), and that uses the ubershader technique to emulate the GPU. This excellent article talks about the what, why, and insanity that are ubershaders: https://dolphin-emu.org/blog/2017/07/30/ubershaders/