List of keyboards with pcb files.
https://github.com/haru-ake/lets-split-pcb-hs-cherry
https://github.com/harshitgoel96/Ergodox_BLE Claims support for Bluemicro and Proton C. Seems incomplete.
https://github.com/zsa/docs
https://github.com/foostan/crkbd Has good documentation. Simple & more complicated version. Supports Elite-C Should be a good starting point to learn. Though much fewer keys.