SHRlKE posted... How does it even work? Like is the whole thing on the knuckles cartridge or does it use assets from what you plug in?
I looked that up a few years ago because I was also curious about it. Here's a pretty good Stack answer on how Sonic & Knuckles "Lock-On" technology works. It's pretty interesting!
https://retrocomputing.stackexchange.com/questions/1514/how-exactly-does-sonic-knuckles-lock-on-technology-work
I'm certainly not well-versed in electronics, but I think I was able to follow most of what the posted answer was saying. Seems like the S&K cartridge attempts to read the plugged in game first to determine if it is Sonic 1, Sonic 2, or Sonic 3.
If the attached game is not one of the Sonic games, then S&K opens up an individual level of Blue Sphere that can be played. If Sonic 1 is plugged in, the game opens up a full Blue Sphere minigame. If Sonic 2 is plugged in, Sonic 2's ROM is mapped to address memory along with a built-in Knuckles in Sonic 2 ROM patch that is present in the S&K cartridge. If Sonic 3 is attached, Sonic 3's ROM and S&K's ROM are both mapped to address memory to create the full game.