people think when one has no problems he/she begins to grow fat or builds up whiles when one takes in more fats and oil without exercise, he / she also increases in weight.
Your curiosity makes me amused. Human physiology and body function is broadly controlled by two different factors: 1) Genetic and 2) Environmental.
For making a person fatty, both of the factors are responsible. If we overlook the genetic factor, the environmental factor responsible for fattiness may be subdivided i) Food ii) Physical labour/ exercise, tension free smooth life style.
Genetic factor someone cannot control but food habit, physical work/ exercise, and smooth life style are human controlled factors for getting fatty among which, food and physical work are vital.
There is some research that suggests that our gut microbiome might also have an influence. See for example the Nature article here https://www.nature.com/ctg/journal/v6/n6/full/ctg201516a.html .
fat is not always bad if stored in the body because they are double energy source than proteins and carbohydrates but saturated fat or the bad fats are a significant risk factor for cardiovascular diseases (CVD) including heart disease and stroke throught raising blood total cholesterol and LDL cholesterol levels.