On Arduino I can set digital pins by setting special variables PORTB and PORTD (or maybe they are not variables?). Where are those variables defined? I grepped /usr/share/arduino, but found nothing.
grep -r 'PORTB' /usr/share/arduino/
PORTB is defined by the "toolchain". It is not defined by Arduino. For instance in my setup the Arduino Uno uses "toolchain" avr-gcc\4.9.2-amtel3.5.3-arduino2\avr.
If you grep that folder (location dependent on tool and installation options) you will find a file called
iom328p.h that defines
#define PORTB _SFR_IO8(0x05)
This is a mapping to a physical address of the cpu.