What makes the brakes squeak and had in shop twice for the brakes

Hello, Brake squeak is normally from loss of anti-rattle clips, glazed (overheated) brake rotors, cheap brake pads, brake rotor warping (more of a moan most times), and replacing the brakes without machining/replacing the brake rotors. What has been done with the brakes since they were new? Also, if a shop helper did the brakes, they may not have cleaned the rotors before applying the new pads, or used a cheap 'stop squeak' to counteract low quality pads or rotors.