It’s part of ArduSub, which is the firmware that runs on the Pixhawk itself. RC_OVERRIDE is part of mavlink, which is the communication protocol for communicating with an ArduSub (or more general ArduPilot) autopilot device. The pixhawk firmware needs to be able to accept mavlink messages as input commands, and send them to update status to external devices like the ground control station (generally QGroundControl for ArduSub devices), but all the actual motor control is done by a PID controller which sends the the desired direction/rotations to the selected frame configuration mapping, which then sends the relevant commands to each motor.
For more of an understanding of how ArduSub works, here’s
- the general ArduPilot Learning the Codebase tutorial
- the motor control overview (from that tutorial)
- the general position control algorithm overview (also from that tutorial)
- the ArduSub stabilize mode code overrides (which builds on the broader ArduCopter stabilize mode code)
Note that the ArduPilot codebase is quite large and interconnected, so it can definitely be difficult to understand all the different components of it and how they fit together. At some point I’m hoping to map out more clearly how ArduSub fits within the ArduPilot code, but I don’t expect that to be done until at least a few months from now, and quite possibly not until the end of the year.