lower intensity cardio results in a greater percentage of energy consumed, coming from fat. Where cardio that is done at a higher intensity, results in a greater total energy consumption, with a lower percentage of that energy coming from fat. However, the total energy coming from fat is greater, at a higher intensity. There is a point, where, as intensity increases, the total energy coming from fat, does not increase. However, training at a high intensity, results in an increase in this threshold.
I hope that answers your questions.
I will add, that I don't think it's that important where the immediate energy comes from. Any glycogen that is consumed will be replaced by your next meal. If you were hypothetically able to burn only fat during your cardio. Then the carbs or protein that you eat, post workout would not be stored as glycogen since your glycogen storage is full, but would be converted to fat. If you were to burn only glycogen during your cardio, then the carbs you eat would be used to replace the glycogen that was consumed.
Say you spend some time on the tread and burn 500 calories. Then consume 500 calories post workout. What's going to happen then? You've just replaced all the energy that your burnt. This energy will then be stored, as glycogen, used to repair muscle damage, and then added to fat storage. In this case the fat that is lost, would be equal to the energy used for muscle repair. Which would be more significant for a beginner but less for an experienced trainee. Since beginners will create greater damage to their muscles from training. You aren't going to lose much, if any fat training this way. Assuming you are eating at maintenance the rest of the day.
Moral of the story is, the laws of physics can't be denied. If you want to lose fat, you must be in a caloric deficit. Use cardio as a way to increse your TDEE, and improve your cardiorespiratory fitness. Control your diet to lose fat.
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