You can NOT lose fat and gain muscle at the SAME TIME!
One of the most common misconceptions that I hear time after time again is you can lose fat and gain muscle at the same time. Often times people think that you can turn muscle into fat. Or people think they feel slimmer even though the scale has not went down so they "assume" they gained muscle.
Let me explain, this is not how any of that works. To lose fat you need to create a calorie deficit through calorie restriction (eating less than your body is burning) for a number of weeks in a row. (Yes, you will lose a little muscle or "lean body mass" during this process as well.) If the scale is not going down, you ARE NOT LOSING ANY FAT. You need that calorie deficit.
In order to build muscle, you need a calorie SURPLUS (eating more food than your body is burning) for a number of weeks in a row (Yes, you will put on a little fat during this process as well) to create new muscle tissue. If the scale is not going up, you ARE NOT GAINING ANY NEW MUSCLE. You need that calorie surplus.
This is why diet is so crucial. No matter if you are trying to lose fat or gain muscle. training should remain EXACTLY THE SAME! The only thing that should switch is your diet and calorie intake. Other misconceptions are, "high reps is to burn fat, while low reps with heavy weight builds muscle." This is also SOOO FALSE. Training should be tracked week by week to gain strength and never changed based on weight loss or weight gain goals.
Do your homework.