I did this for a year and a half in high school.
The McD procedure involves draining the barrel, flushing it with water, then disassembling each barrel assembly and removing every o-ring and gasket.
https://youtu.be/SaYTx7qNJU4?si=Y2ZjWHQJHiKQfAFh
Each part is washed in detergent, rinsed, and sanitized. Rings and gaskets are relubricated with food safe petroleum jelly, then everything is reassembled.
Forget one ring or misalign one gasket and the machine will shoot dairy everywhere when repressurized. Cleanup and rework after that is messy, and probably wasted a lot of other nearby product and disposables. So don't do that.
After reassembly the machine is flushed with sanitizer then clear water then primed with the mix you saved from the beginning. Let's hope you stored it properly in the cooler before it started to spoil. It's expensive. If nothing has exploded it's ready for use after an hour of cooling down.
To do this in 45 minutes with no catastrophic errors is pro-level. And this is why McD went to a self-sanitizing machine.