It depends. At high intensities, your body will burn carbs or muscles before fat. Your body actually prioritizes muscle only when you workout on a regular basis because there is demand for it. Otherwise, your body prioritizes fat over muscle. That’s why fat asses lose their muscle and continue to grow fat. The shit you say doesn’t make sense. If the body prioritized muscles, then you wouldn’t lose muscles over time. Muscles are actually extremely inefficient when it comes to calories/energy. They require a lot of energy to build up and maintain. We wouldn’t have survived if we evolved to be the way you think we are.