QFF is the "true" pressure at sea level corrected from QFE at actual temperature. QNH is a "theoretical" pressure corrected from QFE at ISA. Obviously, QNH can only be "correct" when the conditions are exactly ISA.
Therefore, QFF is used for weather forecasts and charts.
For your ATPL's you're typically asked questions such as which will be higher given certain temps.
A simple aide memoire is
(For airfields above MSL)
Hotter than ISA QNH is higher
Colder (Froid in french) than ISA QFF is higher
Reversed if Airfield below MSL