This video will walk you through the flight controller code of dRehmFlight VTOL to give you a better understanding of the contents and structure. The hope is that it will cover almost everything you need to know in order to feel comfortable navigating the code within the Arduino environment. You can then make your modifications as needed for your custom VTOL aircraft.
The provided documentation on GitHub covers every topic in much more detail and it is recommended to read in full if you are interested in better understanding the code. However after setting up the radio and IMU connections, the majority of code modifications are simply performed within the control mixer function which is where you specify how the motors and servos on your vehicle should move for your aircraft configuration.