Uh, it is very widely understood that eating & absorbing fewer calories than you burn leads to losing weight, and vice versa.

The question is, how does one generally control this?

There appear to be a multiple biological systems, psychological, economic, health, societal, cultural, and evolutionary factors at play which influence how much/how often/what a person eats, how they metabolize what they eat, how much they exercise, etc.

You’re ignoring all that if you try to treat it as matter of “simple physics”. You’re effectively denying the problem by vastly oversimplifying it.