A lot of this based on my experiences doing things like weight training and doing lots of cardio type activities. Also, it is based on the book Advanced Sports Nutrition by Dan Benardot.
From the book, when you do long session of cardio, if you don’t take any nutrients in to match the calories you are burning, your body will burn both fat and muscle as fuel.
From my experience, if you do a lot of cardio, and don’t weight train, you will lose muscle at a much faster rate then in you do weight train. Keep it up for for a while and you will be skinny fat like me.
From the book, if you do take in carbs while doing cardio, your body will burn less muscle. Thus the main protector of muscle during exercise is carbs, not protein.
From my experience, doing cardio to burn fat does not work well since you end up burning muscle also. If you already lift a lot of weights then you don’t need to do cardio to burn calories. After all, why eat, eat, eat for muscle building and then do cardio to burn calories?
Cardio to keep the cardio vascular system healthy may (probably) has benefits for health outside of weight loss. That is, getting on a stationary bike and breathing harder than normal is probably a healthy thing to do.
There are studies that show that sprinting (which is an anaerobic exercise) greatly improves the aerobic system. At the same time it may not hurt much, or even help, muscle growth. Thus look at sprinters who are packed with muscles and long distance runners who have no muscle.
The book Sprint 8 talks about this a lot.
Since I would like to build some muscle while also staying in good cardio shape, I now do sprints on a bicycle. Try sprinting on a stationary bike for 30 seconds going as fast as you can. Then rest as long as you want before doing it again. See how many you can do. Just make sure there is a couch nearby. Studies show a huge benefit from doing just four intervals.