I mean, calories in/out is real, you can’t get fat if you’re eating less than what you’re spending. On the other hand you definitely can thin up eating more calories than you spend by for example going into ketosis where calories don’t matter all that much.

All of that being said, calories in/out is not the whole picture, like you mentioned there are plenty of other stuff that might make it so that two people eating the same and exercising the same amount get drastically opposite results. At the end of the day our bodies have a calorie budget they’re trying to stick to, eating less (or actually eating better) is the solution, exercising helps but not in increasing your calorie budget, only in directing your budget to be more healthy.