What you did next?
What did you do next?
Who your son looks like? You or your husband?
Who does your soon look like? You or your husband?
What did happen next?
What happened next?
Who did build this house?
Who built this house?
Which club did win the most matches?
Which club won the most matches?
Which room does need painting?
Which room needs painting?
Who do you love? I love my wife.
Who loves you? My wife loves me.
Which company do you work for? I work for Google.
Which company produces the best software? Google does.
I hear you did your cooking course yesterday. What did you make?
What made you decide to do a cooking course?
In the interrogative form, an auxiliary is normally required: Who / What / Which did you see? So the forms: Who you saw? What you saw? Which you saw? are not correct. However there are cases when the auxiliary is not required.
Use who / which / what + auxiliary (e.g. Who do / did) when who, which, and what are the object of the verb. In the question Who do you love? the pronoun you is the subject, and who is the object.
Use who / which / what + main verb (e.g. Who says .. Which costs ...) when who, which, and what are the subject of the verb. In the question Who loves you? the pronoun you is the object, and who is the subject.