The base is original from the producer (chords and melodies), but is complemented with elements that complement the song that come from samplepacks, usually, vocal chops, guitars, wobbles sections, glitch effects. It is the standard in which most of producers work, in a project you will find MIDI files made by the producer, but also melodic loops.
For melodical elements I've used BBC Symphony Orchestra, Keyzone Classic, Nexus and guitar loop. Vocal is from royalty free sample pack.
Sample pack Cymatics - Serenity Guitar Loops
Loop name Cymatics - Sinkhole - 130 BPM G Min Main Guitar