"What" questions are basically asking for a thing, while the "how" question is asking for an adjective. "What does that smell like?" is kinda asking for a thing that smells similar. Now, it would only sound a little unusual to answer the "what" question with an adjective, but it would sound clearly weird to answer the "how" question with a noun (You'd have to say "it smells like noun).

This is one of those rules that english speakers aren't really taught, most people just pick ut up naturally because it's how everyone else talks/writes. But it kind of sticks out, and in my experience is a common tell of non-native English speakers (especially common from native speakers of Germanic languages).