How do I see underlying code and definitions inside Arduino IDE on MacOS X El Capitan
The simplest way is to look at the source on Github:


https://github.com/arduino/Arduino/tree/master/hardware/arduino/avr/cores/arduino


Can you set a breakpoint? No. The Arduino IDE and the bootloader don't support such functionality. To do that you need a real IDE (like Atmel Studio) and a hardware debugger.