The PORTs are registers for bi-directional I/O. There are three registers called PORTB, PORTC and PORTD.
The PORTB, PORTC and PORTD are defined in hardware/tools/avr/avr/include/avr/iom328p.h as below.
0x05, 0x07, and 0x0A are I/O addresses of PORTB, PORTC and PORTD respectively. They call _SFR_IO8().
Arduino 1.8.13/avr-gcc 7.3.0-atmel3.6.1-arduino
June 19, 2020