I wouldn't call myself a jazz musician but I have taken some jazz training and I have to say jazz musicians are not just winging it all the time as you think. They are the best musicians in the world in my opinion. While you can be a punk rocker and only know four chords and belt out an out of key melody, you cannot half-ass jazz like that. Maybe there is a lot of improvisation depending on what type of jazz, but they are hardly unprepared. Maybe they break the rules but they learn the rules first in order to know how to best break them 😎