The body burns glycogen stores first. It may take a few hours to a few days for the body to start using fat as a primary source of fuel, depending on the amount of carbs, sugar, etc a person consumes regularly.

Calories in calories out is how fat is lost. You can lose weight eating pizza or donner kebbabs as long as you keep calories at a deficit. Your body would then use glycogen for fuel in this scenario till it ran out then switch to ketones if it depleted.

Ketosis switches your body over to burning ketones for energy instead. It also helps curb appetite and the average person naturally eats less calories, therefore losing weight.

There's no right or wrong way to lose weight but one of the benefits of fasting is to switch to ketosis (see FAQ).