How exactly are movie soundtracks developed

When I listen to my soundtracks from movies I watch, I hear the most irrelevent songs for the movie on there
Is there particular people who snatch good recent songs or do the artists intend to put these, most likely serious songs on a comedy movie (if ya' know what I mean)

Unless the movie scene is a performance that is part of the story, background music is intended to enhance the mood like create the feeling of suspense, happiness or violence for example. The idea is for the viewer to be be focused on the storyline, not the music. Background music is a secondary way to intensify the feeling but it doesn't always work as intended.