The use of an Auxiliary verb

[Rafael]

Hello, I would like to know the reason not to use an auxiliary verb when I ask in this way:
What makes you smile?
Who turned on the fan?
I have a theory about the position of the object and the subject but I would like to know if I'm right. I would apreciate your help. Thank you so much.

[Casiopea]

Welcome, Rafael.

The following are examples of Replacement. Movement isn't a factor: the subjects (Snow, Max) are replaced by WH-words (What, Who):

Statement: Snow makes Max smile.
Question: What makes Max smile?

Statement: Max turned the stove on.
Question: Who turned the stove on.

The same doesn't hold true for objects, or rather, words that come after the verb. Movement and Do-Insertion are factors:

Statement: They eat lunch.
Question: What do they eat?

Here's how it works. The object "lunch" is replaced my "what", then "what" is moved to the front of the sentence, and the auxiliary "do" is added to mark movement, or rather, to signal that a word from within the structure [...] has moved out of that structure, like this,

Statement: They eat lunch.
Stage #1: Replacement => [They eat what]
Stage #2: Movement => what [they eat]
Stage #3: DO-Insertion: what do [they eat]
Question: What do they eat?

Additionally,

With the 3rd person singular, the -s is taken off the verb and added to the auxiliary, giving DO => DOES:

Statement: Max lives on the sofa.
Question: Where does Max live?

With the verb BE (is, are, was, etc) DO-Insertion is not applicable, but Subject-Verb inversion is:

Statement: She is nice.
Question: Is she nice?

Hope that helps.