Need some E.E knowledge's help

Hello guys,
I was told that to control the Voltage coming into a certain product such as ( cellphones that need 6V battery, Laptop that need 12V radio that needs 3V etc ), you need a switch to control the energy collector (or was it capacitor? ) to prevent the products of 6V to receive overcharge.

So i was wondering, is it possible to control it without a switch, automatically perhaps ?

Ps: Not too sure if i explain it clearly,cause i don’t think i understand it either :smiley: