It's stated this way: 1 pound of fat contains 3500 calories.
When you gain and lose weight, you may or may not be gaining or losing fat.
If you
burn 3500 calories, some of those calories will be from carbohydrate (from muscle and liver stores of the compound glycogen), some may be from protein (because it can also serve as fuel), and some from fat.
There's a lot more going on in the body than burning calories. For example, protein is needed to repair muscle, connective tissues, skin... you name it. Fats are needed by all cells for cell membranes, and cholesterol is an important component of cell membranes. Also for skin oils, cushioning around organs, etc. Carbohydrates are mostly used for energy, and most of that goes to feed the brain. Only muscles burn substantial amounts of fat for fuel.
And that is partly why if you burn 3500 calories more than you eat in a week, you should lose a pound a week
on average--but that doesn't mean it will happen every week like that.
Thank you for your attention!
Jay