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.
sample downloaded from the free use looperman. the whole beat creation process is done by me
Sample pack Looperman
Loop name 98s-2smokey UK Drill - Mandolin Melody Made By AV