Modern Vintage Gamer just posted a video explaining how Gamy Boy cartridges work. While this video is also a good explanation of how many similar cartridge-based systems work, the focus is on the original DMG and Game Boy Color, as well as the specific methods Nintendo used to transfer data between the cartridges and console.
It’s easy to think of a console as an “incomplete circuit” and cartridges as the completion of that circuit, including game data and enhancements. While I still find that analogy to be mostly accurate, I enjoyed how MVG presented the more interesting aspects of how this works, as it’s not always as straightforward as “place ROM data on the expected pins”. In fact, his explanation of bank switching made the concept of how the GB/GBC handled large games really easy to visualize.
If you’re interested in the technical workings of the Game Boy, I suggest starting with this video, then checking out MVG’s other Game Goy videos; Next would probably be How Game Boy Graphics Worked, as today’s video almost feels like the prequel to that one: