A hobbyist has a Raspberry Pi, Zero in the case of a Nintendo Gameboy fitted and there is a usb port and bluetooth support is added. The Gameboy is running an emulator and can games be made via micro-sd cards in traditional cartridges.
Hobbyist Wermy freesde two extra holes in his Gameboy enclosure to two original NES-buttons to the existing buttons to add. On the side he placed a usb port for any connection of a keyboard or a mouse, and on the other side, there is micro-usb for charging and mini-hdmi to view on an external screen.
On a remaining usb connector in the enclosure he placed a bluetooth dongle. In an original gamecartridge he placed a memory card reader so that he games via micro-sd cards could be added. The reader was with a few of the pins on the cartridge are connected, and Wermy ensured that when the cartridge holder from the Gameboy to the correct pins on the sd-card-pins of the Raspberry Pi Zero were connected. The mud running RetroPie on the system to the gamecollecties of multiple retro consoles on the Gameboy to be able to run.
Wermy has a gallery of images placed online, and mentions a few details of the parts on the YouTube page.