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.
VST's used to make melodies from scratch are Serum, BBC Symphony Orchestra and Labs. Also, I've used bass loop and 2 synth loops.
Instrument Synth 1
Sample pack Function Loops - Kings Of Psytrance
Loop name FL_PT_KIT02_138_D#_Arp