Why does a car produce white smoke under the hood after the engine is off and the hood is a little hot and the oil has just been changed a few days ago but the temperature gauge is normal?

If this wasn't a problem prior to your oil change, I would venture to say that the oil change technician accidently spilled oil which then made it's way down to engine parts and manifold. It simply gets hot while the engine runs and produces smoke as a result. It will eventually go away as the oil burns off. It is also producing a small amount of smoke while your engine is running, but the combination of the air movement while the car is moving, along with the engine fan, dissapates the smoke below the car so that it is not noticeable until you stop the engine.