I was looking through, generally custom macros are in the config folder, unsure if they’ve implemented it differently, here’s the Raise3D repo I found earlier, klipper has some code in c for the microcontroller stuff AFAIK with klippy in python, I’ve not personally dove into the code, just config and macro stuff largely.

Actually digging through a bit, there’s some gcodes in /klippy/gcode.py in the above repo I don’t see in the Mainline Klipper equivalent, like M9999, it might be a start.