Movie Soundtracks

Most movies you hear have a soundtrack. Whether it's classical music, rap, pop, or anything else. I know for a fact there's a job someone has to choose the perfect songs for the movie. I wonder how hard it must be to choose the songs that fit each moment perfectly. I wonder how long it actually takes, and who needs to approve it. Also, to get the permission from the artist to have it in the movie. It seems like a tedious job, but it also seems fun. I feel like one would have to have an extensive knowledge about music though, which I don't really have.