The usual path was buying an expensive metal box with USB connectors, using logic analyzers ... you can simply use a Pi Pico – you only need to tap the D+ and D- pins, wire them to RP2040 ...

you could have permanently wired the Pico’s pins directly to the Game Boy’s cartridge connector if you wanted to. PICO-GB running on the full-size Pi Pico Then in early June, the RP2040 chip ...