"Which" ("hangi") implies there is a limited selection to choose from, while "what" ("ne") is open-ended. It may help to think of "which" as "which one(s)".
What is your name? (could be literally anything)
Which (one) is your name? (of these name tags; on this list; on the award; in the credits; etc)
What are you doing? (could be literally anything)
Which (one[s]) are you doing? (of the available options/tasks/etc. ex. taking out the garbage or doing the dishes?)