This is a good place to start. Named "the ultimate game boy talk" dives into the CPU used, the instruction set(s) used by the CPU, how the video instruction sets work(ed) etc. The game boy from 1989 is a pretty simple device to start learning from. From there you can look at how various people emulated the system.
https://www.youtube.com/watch?v=HyzD8pNlpwI