The portOutputRegister() is a macro that returns an output port register of the specified port.
TheportOutputRegister() is defined in hardware/arduino/avr/cores/arduino/Arduino.h as below.
The input is P. The macro calls pgm_read_word() with an argument port_to_output_PGM + (P).
The port_to_output_PGM is defined in hardware/arduino/variants/standard/pins_arduino.h(in case of Arduino Uno). It is an array shown below. As this array is declared as PROGMEM, it is placed on flash memory rather than SRAM.
As the port_to_output_PGM is an array, adding (P) to the port_to_output_PGM means the (P+1)th element of the array. In C++ language, the element of array x[i] can be accessed by *(x + i). For example, if the P is PB(2), the third element(note that the elements begins with 0) of the array, that is the address of PORTB, is returned.
PORTB, PORTC and PORTD are registers to execute input or output of digital pins.
The pgm_read_word() is a macro to read a variable defined as PROGMEM.
As a result of portOutputRegister(), it returns the (P+1)th element of port_to_output_PGM that is placed on flash memory.
Arduino AVR core 1.8.3
May 15, 2021