Where is PORTB defined?
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.