The rule "to construct a question, one needs to use an auxiliary verb, such as 'do/does'" does not apply when what is being asked is the subject, because, if what is asked is the subject, it is obviously not possible to have subject-auxiliary inversion (where would we place the auxiliary if the subject is in front position?).
Therefore, questions asking for the subject and introduced by who, what, which, whose, how much/many are followed by the verb as it will appear in the answer:
- What social problems exist in big cities?
Problems such as pollution, crime and unemployment exist in big cities.
Which represents the worst problem?
Pollution represents the worst problem.
How many poeople live in big cities?
Millions of people people live in big cities.
Who lives in a big city?
- I live in a big city.
By the way, in the source you just quoted you can read:
When what, who, which or whose is the subject or part of the subject, we do not use the auxiliary. We use the word order subject + verb:
What fell off the wall? Which horse won? Who bought this? Whose phone rang?