smartphone battery

Many years ago I read somewhere that unplugging your phone without turning the charger off is bad so I have always turned the switch off before unplugging anything. Now I'm looking to buy a power extension board and one of the things it'll power will be my charger. Except most boards don't come...

Let's assume a smartphone is connected to a USB port/charger. When I use that phone, will it consume power from the built-in battery or from the USB port/charger? One day, when the battery life is over, will the phone still work if connected to a USB port/charger?