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 Orcestra, Labs and royalty free piano loop. Vocal is from royalty free sample pack.
Sample pack Boom Bap Next
Loop name 05 Boom Bap Next_Piano_73_Cmin(Harmonic)