Either type of question is derived from a related statement sentence, and in English we have a certain template for how to turn a statement into a question. Some reflection shows that word order is a large component of asking a question in English.
Statement: You want a cat.
Question: What do you want?
Notice that in the question, the order of "you" and "want" is reversed and the word "do" is introduced.
Statement: Yes, you are eating a grapefruit.
Question: Are you eating a grapefruit?
Notice again, that reversing two words signals to the hearer that a question is being asked.
Statement: Bạn muốn một mèo.
Word for word: You want a cat.
Question: Bạn muốn gì?
Word for word: You want what?
The noun "a cat" was replaced with the word for "what" with no other changes to the sentence.
Statement: Vâng, bạn đang ăn một bưởi.
Word for word: Yes, you are eating a grapefruit.
Question: Bạn có đang ăn một bưởi không?
Word for word: You yes are eating a grapefruit no?
The word "có" was added before the verb and the sentence ends with "không."