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.
#define ADMUX _SFR_MEM8(0x7C)
The 0x7C is the address where the ADMUX register exists. It calls the _SFR_MEM8().