Rp2040 boards use PIO serial communication and are connected over 3.5" audio jack.
It uses 3 pins: 3V3 (36), GND (38) and TX (GP0).
Recipe to get it done:
- put rp2040 into bootloader mode
- copy folder to qmk_firmware\keyboards
- compile & flash left side with:
qmk compile -kb custom_b -km default:uf2-split-left
- compile & flash right side with:
qmk compile -kb custom_b -km default:uf2-split-right