Instantly available energy is stored in the body as glycogen (very basic explanation) - when you consume calories, the quick conversion from calories is to glycogen, which is quickly accessible. When this runs out, the body goes to longer term energy storage, which is fat. The body either shuts down functions, which it doesn't really want to do, or it hassles your fat and gets the energy it needs to move about, think, grow hair etc. If it didn't then that's when we feel tired (in this example) or basic body functions get worse. This is how all weight loss works, so your question (if you don't mind me saying) isn't really about Mounjaro it's about how people in a calorific deficit don't feel exhausted all the time. We use the fat stored in our body instead.

I don't know, but I think it's because the body switches from glycogen (instant energy) to fat. I do feel a little tired sometimes, but less than I have done losing weight before mounjaro. I don't feel tired at all, in fact I'm energised massively, although how much this is from the drug vs feeling good about myself and the natural increase in endorphins (etc) I am not sure about.