If I wanted to drive, say, a 1V 0.1A device from mains, I could step down the voltage with a transformer, but if the device was basically just a resistor (overall), then the resistor would have to be fairly large to draw only 0.1A and it would heat up. This heat would indicate that the device is not efficient.
However, my USB wall charger does not heat up, even when left plugged in all day.
I assume something clever is being done with semiconductors, but what?
How can a low-voltage, low-current device be efficiently supplied by mains?