مَنْ فَتَحَ مِصْرَ؟
Who conquered Egypt?
The question seeks the rational agent of the action. An adequate response names or identifies the person concerned.
مَنْ فِي الْبَيْتِ؟
Who is in the house?
Here the unknown is the identity of a person present. Man is an interrogative noun; its syntactic role varies according to the construction, but its normal communicative role here remains identification of a person.
Mā primarily seeks a thing or what it is, whereas man principally asks about a person or other rational being.