Ok here is my logic understanding and experience.
So the body uses carbohydrates as energy. To do this it turns them into glycogen and stores it on the muscle to be used at a later date. Fat is not what you think it is. It is the byproduct of excess calories turned to omentum with the use of insulin naturally secreted from the pancreas. The body doesn't technically classify this as energy for later use. Although many of us will say our belly is a gas tank for the fuck machine. Anywho...
So about losing muscle and how to do it right to lose fat. So think of it this way your body uses up it's glycogen daily and replenishes it with food. So if you go caloric deficient your glycogen stores are depleated. A good way to tell if they are depleted is by checking your weight if you go kito for like 4 days. You see a body has about 2000-3000 grams of glycogen in it. Glycogen needs to be suspended in water in the muscle. So say there is 4000 grams of glycogen and water. Do some math.
3000g/29g=103 oz
103oz/16oz=6.6 lbs
So you should in theory drop around 3-5 lbs as you deplete your glycogen levels down. Now after you do this here is the important part. You can't get the heart going too fast. If you do the body is starved so it goes catabolic and starts looking for any energy source it can find and muscle is the preferred fuel. If you stay nice and low on the bpm like 125-130 the body can burn fat efficiently enough to not be catabolic and your muscle loss should be minimized.
Things that help are steroids, beach chains, protein shakes, and other stuff I'm too lazy to mention. 3j can help with that stuff.
That is how I UNDERSTAND it. I am sure there are things to add to it but I feel like that was parochial for anyone to understand. I get that there are people on this forum who are new and I wanted to make sure they got it too.
Hope this helps. Feedback is welcome!