This isn't a direct answer to your question because I am not OP and I do not know what docs they read but there is a book out called "Game Boy Coding Adventure: Learn Assembly and Master the Original 8-Bit Handheld" that came out last year.
Awesome, I've been getting more into messing with the nuts and bolts of my childhood Gameboy Color, one project I want to eventually do is to recreate it with modern hardware, and then take something similar to GB Studio and embed it into the hardware so I can read cartridges straight to a custom built clone. I've seen some impressive clones already like FPGBC but I would love to build my own. It's a slow burn project, but I also am fascinated by emulators for the platform as well.
