The ADCD/ADCH are registers which store the result of AD conversion.
The ADCD/ADCH are defined in hardware/tools/avr/avr/include/avr/iom328p.h as below.
The 0x78 and 0x79 are the addresses where the ADCL and ADCH registers exist. It calls the _SFR_MEM8().
Arduino 1.8.13/avr-gcc 7.3.0-atmel3.6.1-arduino
June 19, 2020