When you put food in oil it's at s much hight temperature than boiling in water. As a result moisture in the food is boiled out which dies the food. Its also hot enough that browning (maillard reactions) and crisping can occur.

The bubbling when you deep fry is the water coming out of the food and vaporising. You can get pills to boil as well, although they aren't pure liquids boiling is generally accompanied by a lot of smoking, burning of components in the oil, and - because oils are generally flammable - the vaporised fraction often ignites.