When describing the variety or contrast evident in a song we can compare the song sections. Variety adds interest to the song. We ask how is one section different to the next?
I prefer "between" since a comparison requires two or more items to be compared. The first version "how is variety created in these sections...", suggests an internal variety within each section, rather than differences between sections.
There are many ways to create variety within a section. For instance, a section of a song (sometimes called a verse) can have a change in meter (for poetry), or time signature (for music), or an unexpected rhyme structure, such as: A B A B A C B.
There are many ways to create variety between sections as well.
In my opinion, interestingness depends on setting up and then not conforming to expectations. We all have little model builders running while we are listening to a song. One models the melody, another models the harmonies. One listens to the poetry. Each of these models is predicting what will come next. When those predictions are strong, and wrong, our attention snaps to the difference. Most of us like that feeling, and we find it interesting.
Strangely, even when an interesting piece of music that defies expectations is heard repeatedly, it can still give that experience of startlement, and be interesting.
You have variety in a group. You have differences or distinctions between things (or groups).
Example: "Though there's some variety in the pop music genre, the differences between pop and polka are much greater."
So, I think this is most correct: "How is variety created in these sections (of a song)?"