For me at least it's not that simple. My weight (healthy to low BMI) has been very stable over many years with vastly different amounts of exercising and quality of eating habits within that period. Fat to muscle ratio does move but total weight is stuck even with long runs of lots of junk food.
Hmm I have trouble believing that. If you eat less and put yourself into a caloric deficit, your body is going to start using up fat and to a certain degree muscle, that's just how it works and your body is no exception. If you work out during a deficit, your body will try to retain muscle and use more fat instead. Gaining muscle in a deficit is possible, but only in small amounts, if at all.
> quality of eating habits within that period
To me this sounds like you aren't actually changing the amount of calories that you eat, just the quality of them. Eating 2000 calories of McDonalds or 2000 calories of high quality food does not matter (or barely matters) for losing/gaining weight. If 2000 calories is under your TDEE, you can eat 2000 calories of cheesecake and would still lose weight (but very unhealthy).
What I see often is people claiming stuff like "I can't lose weight" or "I have a slow metabolism", but they never actually bother counting how much they eat, and because of that have no understanding of what high-calorie and low-calorie food is. (For example if I'd ask how much calories an average hamburger or a glass of cola has, you'd likely have no idea if you haven't counted calories before).
It was the same for me, once I started counting I realized that I was not eating according to the goals I had set at that time, and that the food I ate was actually much much less in calories than I thought it was.
