The ADCSRA(ADC Control and Status Register A) is a register to control AD conversion.
The ADCSRA is defined in hardware/tools/avr/avr/include/avr/iom328p.h as below.
The 0x7A is the address where the ADCSRA register exists. It calls the _SFR_MEM8().
Arduino 1.8.13/avr-gcc 7.3.0-atmel3.6.1-arduino
June 19, 2020