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.
Synth, lead and reese bass melodies are made from scratch with Serum VST and also I've used bassline and plucks loop.
Sample pack Prime Loops - EDM Essentials Trap
Loop name Bpm70_G#_Wals