when exactly django query execution occures

QuerySets are not evaluated until you do something that actually needs them to be evaluated. As the documentation for the class itself states a QuerySet:

Represent a lazy database lookup for a set of objects.

Emphasis on the word lazy. This is because one often needs to call or chain methods on a queryset, a good example being a group by requiring subsequent calls to .values() and .annotate(). If a queryset was evaluated directly then we would be making too many unneeded queries to the database, slowing down execution to a halt.

As to when exactly a queryset is evaluated I would list the answer in short (for the long answer refer to When QuerySets are evaluated [Django docs]):

• Iterating a queryset
• Slicing a queryset (with the step parameter)
• Pickling/Caching a queryset
• Calling repr(), len(), list(), or bool() on a queryset
• Various methods like get(), first(), last(), latest(), or earliest(), etc. also make a query to the database