You realize those two ideas are at odds, right? If you're at a calorie deficit, and your body recognizes that, even thinks it's in a starvation emergency, it won't do both. The myth is that if you're not eating enough your body will refuse to let go of fat (stored energy) so it won't die of starvation. Now if it's going to those extremes, it's not also going build muscle, which is a metabolically expensive process, and results in tissue that needs more energy to maintain than fat.

As a general rule (there are exceptions, but relatively narrow ones) you don't build muscle at a calorie deficit. You need an energy surplus, and enough protein and carbohydrate, plus the right hormone environment to build significant muscle tissue. That's not going to happen while your body thinks it's dying of starvation. Sadly, muscles aren't made of pixie dust and wishful thinking.