Intel CPUs are well document both by their documentation or by third party docs (like Patterson-Hennessy.) Although NVIDIA is less document than these CPUs, there are some well-written whitepapers including one for academic conferences. And these are the basis for the textbook coverage.

Compared to that, ARM or Qualcomm GPUs are totally a blackbox. I'd love to learn the specifics of mobile GPUs but there is no clue. I wonder how Collabora folks have figure out the instruction set architecture and other details.