Changing or Adding folders,codes to ArduPlane

Hi, have a nice day.
I have a some question about developer side.
Right now I have a 4 tilt motor quadplane. Naturally, I am using ArduPlane, but my plan is to implement ROS based AI, extra sensor data comparison in ROS and I want to implement some commands that can be sent from ROS to Controller.
In this manner, I search and some of the developers ( really thank you) came to me some answers but I am newbie so I need more detail.
I want to change and implement copter codes (e.g precision landing) I have an intermediate understanding written codes to become more expert I am still imrpoving myself BUT right now I have to start to change and implement the codes to ArduPlane.
So for example if I wanna make landing with QR code in ROS, do I have to add command changes motor speed, control surfaces or if I write the code about landing with lidar and camera , then just send the distance etc. can ardupilot handle the other situations? I am asking that because ArduPlane doesn’t have mode called as precision landing.
If I don’t need to add this mode in master, is distance ROS code enough?
If it is not what part of the code do I have to be more attentive to send the motor speeds, PIDs etc.
Also, where do I have to add specific folders in master if I make a change or which folders…
I need a brief for developing.
I know they are very basic and as a developer I don’t have to ask but I am in right now and I don’t know which course can teach me to make them.
If there is a document about hole and specifically ArduPilot( maybe more general makes me think wrong, some of the folders meaning can be different.) could you drop here?

Thanks a lot if you spend your time answering me.
Have a nice day.