A carburator is like a valve in a way. It draws in air to feed engine a fuel and air mixture controlled by a plate that is the same diameter as carb. The plate when closed decreases the amount of air entering engine. Fuel is introduced through small orifices, air pulls out metered amounts through jets into the air stream and mixing it before entering engine. Control is done by the butterfly plate or a sliding barrel with a tapered needle that controls fuel metering. Usually sits in the middle of carb. Hard to explain but easy to understand when you strip one down! Throttle bodies also control the air entering much the same way but fuel injection is sprayed into air stream and metered by the ECU using sensors that are informing injectors to meter fuel amount by pulses. Throttle position sensor tells ECU how much throttle is being opened along with air sensor that measures amount of air entering engine. Other sensor inputs also make sure air/fuel ratios are in the parameters set. Explanation is not easy to put into words but I hope some of it helps to explain a bit More?