I started with https://docs.djangoproject.com/en/4.0/intro/tutorial01/
In the models.py file (in the polls app) I ran through the following documentation and documented my knowledge in there:
https://docs.djangoproject.com/en/4.0/topics/db/aggregation/
https://docs.djangoproject.com/en/4.0/topics/db/queries/
https://docs.djangoproject.com/en/4.0/ref/models/expressions/