How onStart() is calling after onCreate() method?

I am learning about design patterns in java. In Android, every time I read that, first Oncreate() is called and next onStart() and then onResume() called . How they are calling internally ? Which design pattern they are using ? How these Callback functions are implemented ? were they used observer pattern or interfaces or abstract classes ? How onStart() is calling after onCreate() method ?

I read the source code, But I didn't understand. Please explain me once

How onStart() is calling after onCreate() function and which design pattern ? Please give suggestions, How should I read source code ?