In one sense, the basics of Tin Pan Alley have not changed in a century: search a music library (now online), choose your songs, and pay the composer or the library or both. It is just that now stock music sites have gone global with a big presence online and vast collections. Their music is sold not only to the movie industry but to producers of the widest spectrum of videos from marketing to TV series to educational and training to YouTube personal and commercial productions.
Despite the diverse uses of production music, a broad distinction is still drawn between what music publishers work with composers to bring to market and music largely created for music libraries by mostly work-for-hire musicians who do not own any rights to the work and usually remain behind the scenes. Not that the two fields don't overlap, of course; but when a big-budget movie wants to use a hit song there is no "free" or "cheap." We might be talking tens of thousands of dollars.
And, of course, big-budget movies may commission music directly from top composers and independent filmmakers may work with musicians just beginning their careers.
None of that means we can generalize with confidence about "production" music versus "performer" or "creative" or "artistic" music. There is huge talent on both sides of the divide.
What is relevant is that, given the way most "production" music is created, the issue of licensing use of a piece of such music is simplified. Most often, the stock music site owns it outright; no royalty is involved.
Partly because of today's astonishing range of videos being created, almost always including a soundtrack, production music libraries bring together a wide range of styles and genres – often thousands of tracks – and facilitate searches in various ways. Usually, there are search filters for genre, mood, type of instrument or voice, and more. Some libraries make specialty teams who take your "specs" for the music you need and do the search for you, producing options.
What very few video producers or video fans doubt is that music can "make" your video and if you are watching your budget this may not be the place to save money by using the second or third best. Mood and impact have virtually "created" the indelible impression of movies, TV-series, commercials, and brand associations. "Stars Wars" can be heard coming a mile away. And music theme songs from Westerns made four decades ago still play in our heads. And for baby boomers, how about "Davy Crockett, King of the Wild Frontier"?
Stock music sites in 2020 can be free, very economical, a necessary investment, or plain expensive. That isn't always due to the quality of the music, per se. Other factors might influence the philosophy of the site (help the best young talent achieve recognition), the auspices (profit or nonprofit), use of music (commercial or not), amount of service requested, or volume of your purchases.