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 combined 2 samples on Loopcloud. Then i made the drums and 808.
Sample pack Trap CEO
Loop name Prospect_145_Bpm_Dm_Guitar_Major_Loops