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.
I found a xylophone sample so I started cutting and adding some effects, after creating the drums and adding some ambiences, the result was a completely different sound from the original sample.
Sample pack Ghosthack - Cinematic Orchestra
Loop name GH_FS_Celesta & Xylophone_ 5_80_Fmajor_Celesta