Why is the 'what' put at first?

Question words like "What, Who, Where, Which, How.." are placed at the beginning of a question and the question mark (?) at the end.
"What did you cook?"
"Who is your teacher?"
"Where can you eat cheaply"
"Which book are you using?"
"How are you?"
Without question words:
"Did you read the book?"
"Can you speak English?"
"Do you play golf?"
"Have you got a c

Then what about this case? "Do you know who she is?" This is what I meant. In this case, who isn't put at the head. Then why is the what in the example put in front of the sentence. That's what I'm wondering. Would you add some more explanation?

Here in India, we call it W-H family

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"Do you know" is the question form. For example:
"Do you know Mary?" "I know Mary."
"Do you know a lot of people?" "I know a lot of people."
"Do you know who she is?" "I know who she is."

"what" is the information you want to know

What I want to know is, as in the case "Do you know who she is?", can the example be replaced into "Did you say what you've got at home?"

Yes, it's very wonderful. In the example, the one who is asking is not want 'yes or no,' he's just wondering, so 'what' is to put at front! Thank you very much.

eipjooWhat did you say you’ve got at home, Malfoy, a Comet Two Sixty?

This is two questions in one. Only the second one, which suggests the answer to the first question, is a 'yes-no' question.

What did you say you've got at home?
Is it a Comet Two Sixty?

These can be punctuated differently.

What did you say you