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.
Most of the bass drop sounds come from one shots and loops of various sample packs, it also includes a serum bassline and lead.
Instrument Lead Drop
Sample pack Slap & Bass House Drops
Loop name SHD Construction Kit 03 Drop Lead Loop G# 126