ATPL - The QNH is by definition the value of the:

Question id 116 - Aircraft General Knowledge - Instrumentation - ATPL - Airline Transport Pilot license

Atmospheric pressure at the location for which it is given, corrected for non-standard temperature.

Altimeter setting so that the altimeter, on the apron of the aerodrome for which it is given, reads zero.

Altimeter setting so that the altimeter, on the apron of the aerodrome for which it is given, reads the elevation.

Atmospheric pressure at the level of the ground overflown by the aircraft.