Tutorial about the source code of PX4

I want to learn the source code of PX4, but I don’t konw how to begin.At the moment, I just read the source code aimlessly. I want to read and modify the part of control, making my drone fly in the sky. I read the tutorial about wrinting a “hello sky” on the website, but I failed to find a tutorial, which teaches me how to read the code of control, and how to modify it. Can anybody offer me a way to make it? Thank you very much!

Hey, check this