No auxiliary verb in a question?

When we learn about interrogatives, we are told that the correct word order is auxiliary verb + subject + infinitive (A-S-I). There may be (puede haber) other words, such as an direct or indirect object, an adverb, etc., but the auxiliary must precede the subject and the subject must precede the infinitive:

Do you smoke?                                              A= do    S= you    I= smoke
Did your mother shout?                    A= did   S= your mother   I= shout
Where does she live?                                 A= does    S= she    I= live
Why doesn't the baby always eat fruit?     A= doesn't   S= the baby   I= eat
What does "litter" mean?                       A= does    S= "litter"   I= mean
What does mean "litter"?                        incorrect word order
What means "litter"?                                incorrect: no auxiliary

However, there are some questions where this does not happen:

How many people came to the party?
Who eats fruit every day?
Which of these animals lives in the sea?
What happened then?

Why? Because the WH- words at the beginning of the sentence (how many people, who, which of these animals, what) are the subjet of the question. You start your sentence with the subject, so it is imposiible to follow tha A-S-I rule. In these cases you don't use the auxiliary verb. The word order is the same as in positive sentences: Subject + verb + (...):
How many people came? Twenty people came. (Subjects: how many people and twenty people).
Who eats fruit every day? Pete eats fruit every day. (Subjects: who and Pete).
Which of these animals lives in the sea? The dolphin. (Subjects: which of these animals and the dolphin).
What happened then? Nothing happened. (Subjects: what and happened).

Compare these two sentences:
Who did you see? I saw my boss. Who= direct object, and then A-S-I. You is the subject. My boss = object.
Who saw you? Nobody saw me. Who and nobody = subjects. You and me = objects.