Pop music is of course used frequently as a backdrop in many films. It may be very effective in evoking nostalgia for a particular era, like in Forest Gump. It may be commissioned especially for a film, as in some of the tracks for William Shakespeare's Romeo and Juliet. On the other hand, there is a big trend to include more and more songs in film soundtracks for marketing purposes, and there comes a point when the film can become a quasi promo video for bands and singers. As a rule, song in films tends to divert the attention of the audience away from the narrative, which may or may not be what a director has intended. Some movies such as Easy Rider, Trainspotting and Zabriskie Point, had no underscoring at all, relying entirely on pop songs which tended to add to their artistic and commercial success. These are films about youth culture and what could be more important and revealing for young people than rock songs? Certainly not  Hollywood style  string
music !

If a film has one recognizable melody or song, it is generally at the beginning and or at the end. However, films often have more than one theme tune. For example, Once upon a time in the west has a harmonica riff for Charles Bronson, a jaunty banjo piece for Jason Robards, and a rich orchestral melody for "the land". Serge Leone is one of the very few directors who actually used to cut his film to the music which Ennio Morricone wrote for him.
Sometimes during the opening sequence/credits the composer has crammed into one piece the most memorable themes that are going to be used later in the film as underscore. A great master of this technique is Elmer Bernstein.

Music supervisors take a close involvement in the choice of composer, and may liaise between director, producer and composer. If a film has predominantly songs in the soundtrack, it will be the music supervisor's job to research and help choose which songs finally make it into the film.
They can also be in charge of organizing the actual recording of the music and hire conductors, orchestrators, arrangers, musicians, studios etc... and sack the whole lot as Bernard Herrmann used to do sometimes!

Film editors cut the film and work in close relation with the composer.

Music editors keep tracks of the places where music is to be brought in. All these informations about reels and timings are included in what is sometimes called a film breakdown sheet that lists all the different cues.They also prepare cue sheets which present short descriptions of the events (with their timings) happening  in a film sequence (a cue). In some cases music editorscut and edit the music to fit the scenes . Some composers like Vangelis compose music without precise timings and it is up to the music editor to have the music fitting the film.