Dang, this guide by Allison Parrish is a great example of technical documentation: easy to read assuming you know at least a tiny bit about the subject of how software talks to hardware, while still being thorough in its scope and links to more reading (the info might indeed be more than you wanted to know, but hey I appreciated it).
It’s fascinating to me that the games of that era contained so much hardware unique to each release packed into a form factor as tiny as a GB cartridge, while still being reasonably affordable (not to mention the ones that went further with custom on-cartridge peripherals like accelerometers and vibrating rumble packs!). Modern games just shipping on SD-type memory cards seems so simple by comparison.
Go learn more than you wanted to know (and then maybe fire up a round of Tetris)…