Why is smart DC adapter/charger voltage far lower than its rated output one when it's open, not plugged for charging, how is actually the way it works

 

Are you talking about a USB device?

 

It maybe is outputting a short pulse to see if the battery is there. Can you look at it with an oscilloscope?

 

It goes to idle and will start up when it detects a load. That's why it is called "smart"

 

Goes to 'sleep' mode while not in active charging to not waste energy. People likes to leave them plugged on the wall outlet after the recharged device has been taken for use; and these chargers were invented to not consume much current in that asleep state.