The majority of music featured in films are sourced by sync agents. These agents will have a pool of artists, labels, managers etc that ‘pitch’ the music to the producers/directors of the movies for what's called ‘placement' (this is also very common in advertising and TV).
For musical score, although more established production houses have their preferred composers, smaller independent filmmakers are worth contacting but don't expect to get paid. Look out for small budget movies in production on social media (Twitter is good for this, follow lesser known, up-coming actors for the roles they will be playing). This way, if you land one or two of those, you can build your portfolio.