Your first sentence I don’t believe for one second is always true. There are plenty of overweight people who don’t overeat and really thin folks who eat whatever they want. It’s down to ones genes.
There is only ONE way to get fat. That is to consume more calories than you burn. That’s it. If you are fat, you ate more than you needed to. That is a choice. If you don’t want to be fat, you need to eat less or burn more calories. Preferably both. Thin people can get fat. Fat people can get thin. To say anything else demonstrates complete ignorance about biology.