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.
Synths, sub bass, supersaw bass and FXs are made with Serum VST. For violin melody I've used BBC Symphony Orchestra VST.
I've also used bass loop and vocal from sample packs.
Sample pack HY2ROGEN - Make It Slap
Loop name HMIS_Kit14_Synth_Mid_Bass_124