The DDR(Data Direction Register)s are registers which determine if the digital pin is output mode or input mode. There are three registers called DDRB, DDRC and DDRD.
The DDRB, DDRC and DDRD are defined in hardware/tools/avr/avr/include/avr/iom328p.h as below.
0x04, 0x07, and 0x0A are I/O addresses of DDRB, DDRC and DDRD respectively. They call _SFR_IO8().
Arduino 1.8.13/avr-gcc 7.3.0-atmel3.6.1-arduino
June 19, 2020