Some direction on how to use MAVROS with a pixhawk 2.4.8 flashed with ardusub?

I have flashed Ardusub Firmware onto a pixhawk 2.4.8.
I also have ROS Noetic installed on a laptop with Ubuntu 20.04, and have ran the apm.launch command, which runs with no issues.
The question - Now what? How do we go from here to an AUV? We are trying to test run a BLDC via the pixhawk to see if it works? What commands do we run for this? How do we write the python code? Any direction from here would be extremely useful.
The goal is to make an AUV using a raspberry pi and pixhawk, and I am very confused as to how to proceed here.