Are any home gadget that is written "smart" on the packaging, capable to interact with a microcontroller through some protocol or "smart" label is just another marketing ploy?
No. For example:
- I have a bread making machine which is smart but has no external interface.
- I have a Sony hi-fi that is not intended to be controlled by something else but if you can get the Arduino to generate the infrared remote control codes then you could send commands to it.
Sometimes it is possible to simulate a button press on a device using a relay or transistor to short out the button. This requires a level of understanding of the circuit being controlled so that the control works and you don't damage the device.