Who compose the copyright-free music to use in movies and/or other things

[Drealm]

I know that there's a great amount of music available free of copyright when you want to do, for example, a movie but can't afford a Michael Jackson song in it.

So I was wondering, who compose this music?!!

Is it composed especially for this purpose by some unknown composers?

Or maybe it's just music that got lost in time so they belong to anyone, like some "public domain" movies.

If someone can bring some light...