The SERVO settings are what you are after.
The RC settings are inputs from the reciever, and SERVO settings are outputs to motors and similar.
For AUX1 output use SERVO9_FUNCTION
To set it to mirror a RC input, set SERVO9_FUNCTION,(51 to 66) to suit your RC input channel.
Setting SERVO9_FUNCTION to value 1 (RC Passthrough) would pass RC channel 9 to servo9 output. RC Passthrough would rarely be used - it’d be better to set the specific RC channel.
If you’ve got some specific function in mind, like Camera tilt, Landing Gear or Sprayer - then better to set that function and let the flight controller operate it under certain conditions, so you dont need to be flicking a switch on your transmitter unnecessarily. Works better in Auto mode for missions too.
You’d obviously also need to set up other associated parameters for that function.