Hi @Sarrus4x4 - that’s a good and in depth question 
There’s a number of different ways and places this has and can be documented. Since BJS is a wrapper over WebGL/GPU (shhh! Native is another discussion!), the WebGL specification would naturally have insights into the pixel processing stage, as that mostly happens on the GPU in the form of pixel shaders. You could also go straight to the source (as it were) and browse the BJS source repos to see for yourself how the rendering pipeline works internally. The code is fairly well documented with comments, but that could be too “low-level” for what you want.
In my book, I’ve got a couple of chapters devoted to exploring topics like how lighting calculations work, what goes on inside materials, and how rendering pipelines work with different types of shaders.
Since you’re asking for very specific info on the BJS rendering pipeline, I’m unsure if my book would serve to answer your questions. It could be a start though!
Ed: updated BJS repos link to point at engine src