How can I learn PX4 software better?

Hello everyone, I want to understand the PX4 system. My goal is to understand the system better and write my own Airframe. In the system I want to write, there is a motor connected to the FMU port for 4 drones to fly. And there are 2 motors connected to the IO port for the rover part, apart from these, I need to drive 3 servos for camera control. My goal is that while the drone is disarmed, the rover part will be activated and the commands sent to the engines used for the rover part will be sent. I want the rover part to disarm when the drone enters the arm state. What kind of progress should I make with this code? How can I learn the system better?