That's all fine and well, 1dwing, but for different people different foods make sticking to that deficit much easier. For some it's low carb, for others it's low fat, for others they might have some interaction with a disease or some drugs they're taking. For that reason it's important to present as many viable options as possible. There's no one plan that works for everybody. So yes, it's calories in vs calories out in theory but saying it's only that grossly oversimplifies the situation.
1) eat real food - more vegetables, moderate meat, moderate fruits, less grains, less sugar, less vegetable oils.
2) exercise - moderate intensity cardio, sprinting, heavy lifting, dedicated stretching and mobility.
3) live - relax, de-stress, meditate.
Disclaimer: I'm not professionally qualified to make any formal recommendations. I've just done my homework and I'm my own guinea pig. All of my data, unless otherwise cited, comes from a sample size of n=1 (me).