About the Brisk method

Hello Rabbi Michael,
Rabbi Chaim of Brisk is known for saying that in learning, “we do not ask why, but what.” That is, in theoretical inquiry, they do not ask why there is an obligation or a law, but rather what the essence of the obligation or law is (or in Yiddish: “مان درف فرستن واس عس شتيط, نيت فراوش”).
But I’m having a bit of trouble understanding. If we take, for example, the obligation to eat in the sukkah on the first night, Brisk will surely investigate whether the obligation is the very fulfillment of the mitzvah and there is an independent mitzvah to eat in the sukkah on the first night, or whether eating is a clause of ‘sit as you would a regular meal’ and the dwelling is the mitzvah and eating in the sukkah is merely its expression.
But even in this “investigation” to find the “what” I would first have to ask “why,” meaning why in general a person would be obligated to eat on the night of Sukkot (apart from the issue of the equal derivation from the Feast of Unleavened Bread), or I would formulate it from “where did the issue of eating on the first night in the Sukkah come from,” and then I would also arrive at the “what,” which is deeper. It is difficult for me to see how one can reach the essence of the law/obligation with a question of only what? After all, “what” only investigates what is before us, not where it came from and its essence.
Additionally, in the way you study, do you identify with this form of study? And do you think this is also the way of study of Rabbi Shimon Shekap and a few others, or is it something that is unique to Brisk?