I'm building my first robot the BMO. Did you want me to help? do you know where to solder a game boy cartridge entrance on the Raspberry Pi 3 plate?
I want to make it like this video:
But with a game boy cartridge slot.
You're missing the video link! In any case, the RPi3 has no test/solder pads for the SD card, so to connect it to the cartridge slot without desoldering the SD card slot is to crack open one of these adapters, and solder it to the cartridge slot like in Wermy's original guide.
Unfortunately the Pi3 does not have SD card solder pads like the Pi0 does. Using the adapter or desoldering the original SD card socket is the only way.