Where are the Serial functions in Arduino defined?

In which file are the Serial functions like .begin(), .flush(), and all the others referenced here arduino.cc/en/Reference/Serial actually defined? In AndroidStudio, you can just Control+Click on a method and it opens the file where that method is defined. Why do things have to be so difficult with the Arduino IDE? Is there an alternative IDE for Arduino that enables one to delve deeply into the arduino core without spending hours searching for the files where things are defined.