Most questions ask for the object of a sentence.
SUBJECT VERB OBJECT
Lee Oswald shot President Kennedy.
Who did Lee Oswald shoot? ANSWER = OBJECT (President Kennedy).
With the question words WHO, WHAT & WHICH, if the answer is the SUBJECT, there is NO AUXILIARY 'DO, DOES, DID and the WORD ORDER IS THE SAME AS A STATEMENT.
Who shot President Kennedy? ANSWER = SUBJECT (Lee Oswald).
Here are some more examples of subject questions:
SUBJECT (+ VERB + OBJECT)
Who broke the window? Peter (broke the window)
Who discovered America? Columbus (discovered America)
Which actors starred in Casablanca? Humphrey Bogart & Lauren Bacall (starred in Casablanca).
Which switch operates this machine? The red switch (operates the machine).
What happened to you last night? Something terrible (happened to me last night).
In contrast, here are the object questions for the examples above:
What did Peter break?
Which continent did Columbus discover?
Which actors did Casablanca have in it?
What does this switch operate?
What did you do last night?