First of all, that nutritional precision thing? It doesn’t actually exist. You have literally no possible way of calculating exactly how many calories your body uses/needs. It’s different every day and is influenced by an infinite variety of factors. And, you have literally no possible way of calculating exactly how many calories are in food, because the nutritional content between things is never exactly uniform. The living world cannot be calculated to precision. Even with heart rate monitors and food scales, the closest thing we have is still an educated estimate.

To build muscle, your body needs to be receiving enough material (food) to be able to construct new tissue. When losing weight, you are receiving less than maintenance amounts of food, therefore deliberately putting your body in a state of existence where adding new mass is not possible. “Something” (muscle) can’t be created out of “nothing” (negative net calories). That is literally a law of physics.

Generally, the only people who experience fat loss while gaining muscle are people who are new to gaining/muscle building. This is usually a result of their body having no choice but to burn fat, to keep up with their newly elevated metabolisms (which is achieved by gaining muscle). However, this is a change in body fat, not a loss of “weight”. Muscle is gained and fat is lost.

After the initial gain, usually a body won’t respond that way anymore. This is why people go through bulk and cut cycles. You commit to building muscle and gaining a little fat, then you commit to slowly losing the fat while retaining the muscle. Attempting to do both at once is a whole lot of effort for substandard results.