An educated guess (having coded the NES) is that it's simply a bigger ROM, and the cart contained some kind of MMC chip [0] which allows different sections from

An educated guess (having coded the NES) is that it's simply a bigger ROM, and the cart contained some kind of MMC chip [0] which allows different sections from the ROM to be paged-in - the menu screen contains the code to page-in the applicable bank from the ROM, and then the game runs per normal, not being aware of any of this. [0] https://en.wikipedia.org/wiki/Memory_management_controller https://en.wikipedia.org/wiki/Memory_management_controller