Realised I forgot to respond to your earlier comment.
I’d suggest looking through ArduSub’s control_*.cpp files, and possibly the general AC_AttitudeControl_Sub.* and AC_PosControl_Sub.* files.
This may also be useful:
While just testing in water is best, for bench testing you can either adjust the thruster limits or unplug the ESC signal wires and just look at / print the SERVO_OUTPUT_RAW values.
Correct, each flight mode has its own input handling, and there’s no built in overriding behaviour for the outputs that bypasses the flight mode’s intended actions, so if you want a flight mode that stabilises normally but treats inputs as manual overrides then you’ll need to write that as custom.
The RC_OVERRIDES terminology refers to overriding the inputs, namely from actual radio control channels, transferred in via the “RC” input (expected to be from a radio receiver, which is how other vehicle types are often controlled).