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 LABS, Keyzone Classic, Nexus and vibraphone loop.
Sample pack Kryptic Samples - Samples To Slaughter
Loop name S2S2-VIBES VIBRAPHONE