unity has an extremely well-designed abstraction layer over the whole graphics API, exposing pretty much everything the platforms support with minimal boilerplate. It's comprehensive enough that URP/HDRP are built on top of this layer; they don't need to drop down to the closed-source C++ secret-sauce part to make full use of the GPU
like, if you want to run a compute shader, you just put it in a .hlsl file and fire it off with like 20 lines of code
