The short explanation is your body acquires the calories it needs to build muscle through the fat stores.

When the body is deficient in caloric needs, it will burn body fat to make up for the difference. Say you need 2500 calories a day to stay the same weight, if you consume only 2000 calories a day you will lose weight. The body will acquire the calorie difference through the fat stores and can use more calories to build muscle as well.

It’s just been a old myth that the body can only perform one or the other when it comes to fitness goals but it’s quite fascinating how the body can achieve both fat burning and muscle building.