MidiCard is available for purchase at: https://makerproducts.art/product/midicard-plus-25-key-small-midi-piano-keyboard-controller/
This is the Arduino code for MidiCard, a tiny PCB USB C MIDI keyboard powered by a Seeed Studio Xiao SAMD21 with 25 multiplexed buttons - pins 0-4 are input, pins 6-10 are output. The board has 18 keys, and function buttons for OCT-, OCT+, SUS, MOD, P, MF, and FF.
https://wiki.seeedstudio.com/Seeeduino-XIAO/
The plusv1.0 branch has the code for the 25-key MidiCard Plus, which uses a bare SAMD21G18A chip.