MJ couldn't fluently play any instruments so he would sing and beatbox demos of how he would want his song to sound on tape.
One of his engineers Robmix on how Jackson worked: “One morning MJ came in with a new song he had written overnight. We called in a guitar player, and Michael sang every note of every chord to him. “here’s the first chord first note, second note, third note. Here’s the second chord first note, second note, third note”, etc., etc. We then witnessed him giving the most heartfelt and profound vocal performance, live in the control room through an SM57. He would sing us an entire string arrangement, every part. Steve Porcaro once told me he witnessed MJ doing that with the string section in the room. Had it all in his head, harmony and everything. Not just little eight bar loop ideas. he would actually sing the entire arrangement into a micro-cassette recorder complete with stops and fills.”