Humans are no different than other mammals.

Look at overweight house cats and dogs. How did they get overweight? How do they lose weight?

Calories in / Calories out is the definition of survival or not for wild mammals.

Calories out is also exceedingly easy to calculate- on any given week if you gain weight the out is less than the in. If you maintain its ahout equal. If you lose weight out is higher than in.

Nothing else matters.