The ADMUX(ADC Multiplexer Selection Register) is a register that controls the reference voltage, the presentation of the ADC conversion(left adjust or right adjust) and analog channel selection.
The ADMUX is defined in hardware/tools/avr/avr/include/avr/iom328p.h as below.
The 0x7C is the address where the ADMUX register exists. It calls the _SFR_MEM8().
Arduino 1.8.13/avr-gcc 7.3.0-atmel3.6.1-arduino
June 19, 2020