i used to submit for licensing to film/tv...

from my (limited) experience, there are usually 2 scenarios:

1) there are agencies who act as middle men between the labels/artists and the film production companies. The artists/labels/managers submit large batches of music (usually they request sample free stuff) to the agencies. The agencies put them into this large database of music. The production companies go through the databases and pick what they like, kind of like ordering off a menu. The publishing eventually trickles down to the artist, but there is usually no contact between artist and production companies. The agencies essentially lobby the prod companies to have their batch considered over others. The agency's relationship with the prod company is key to getting their client's placement.

2) There is some kid who works on a film, but secretly stans for indie artist X. He convinces a director or whoever to include their music in the film. The licensing process happens in reverse, in the sense that the prod company now reaches out to the label/artist, etc.

there are exceptions, but this is pretty much how it works. Truthfully, i stopped fucking with it cuz I got bored doing non-sampled beats in this sort of factory-style way...your music ends up in database of a gazillion songs. and, it is literally a crap shoot in terms of getting picked. I think i got a few beats in some commercialas i never even saw. To this day, don't know much about it, only the ASCAP check says so....