Would like to know about PID tuning of drones in detail

I would like to know more about PID. The math behind it, the calculations, the parameters, everything in detail. We know we can change the PID values on Mission Planner by observing the flight. That is what a customer does. But from a developer point of view I would like to know everything about drone PID.

You might be able to find PID tuning videos on youtube. It is a big field. Essentially, PID controller uses sensor path data, the intended path data, and their differences to correct the path to what you want. P is proportional gain, I is integral gain, and D is derivative gain. This is a good introduction: PID Control - A brief introduction - YouTube