This is one of the most common forms of appropriation and recomposition I’ve seen on Youtube. Fans of various shows/movies/books/etc. will often take songs that they think fit the storyline or the feel of said show/movie/book (or that fit what they think the storyline SHOULD’VE been like), then use clips from them to make new music videos. (In the anime world these are well known as anime music videos or AMVs.)
These music videos vary greatly in quality. The can be a tremendous flop, or they can be absolutely beautiful, like this video here.
(Not gonna lie, I’m obsessed with this video. I love this song, and I love Harry Potter, and this is done so well I can’t even put it into words.)
And here’s another fan-made video (a parody, making fun of the popular ninja-anime Naruto) that’s really just stupid-funny. It’s not really a music video, but it does mention AMVs towards the end.