I personally know people who are very smart, but listen to simple music, as well as very smart people who even create very simple music, and vice versa. There is no direct or feedback here. If you listen to avant-garde music, you won't get any smarter from it, and if you add books on quantum physics, you won't be able to listen to other music either. But the quality of listening to music, how a person perceives this music, what thought processes occur while listening, this is a completely different conversation that goes beyond preferences.
Yes, people with a high IQ most often listen to matcore and similar genres, while people with low IQ tend to listen to easy-to-understand genres (chanson, pop music).
Definitely not. There are, of course, some correlations between musical taste and IQ, but the style of music influences this to the least degree.
In general, statistical studies show that people who like "vanilla" meaningless quotes have an IQ lower than those who distinguish complete nonsense from "beautiful" text. What does this have to do with it, you ask? Well, despite the fact that there are songs that consist of such quotes and simply do not carry any meaning, but some people like it. Most often, of course, just because of the rhythm, they do not pay attention to the meaning, so it is stupid in itself to consider a person smart / stupid because of musical preferences.
So is there a connection or not ? As I said, no. No. Not a drop. Music does not affect the level of intelligence in any way. But on the contrary - yes, the level of intelligence often directly or indirectly affects musical preferences.
You can also read about the refutation of the usefulness of classical music for the intellect. It's right here.