What is the name of the item that supplies the exact or near exact voltage at the required wattage to all of the circuitry inside your computer?

It's the power supply

Explanation:

The power supply is what essentially enables the computer to operate. It is able to do that by converting the incoming alternating current (AC) to direct current (DC) at the correct wattage rating that is required by the computer to function. The power supply is a metal box that is generally placed in the corner of the case.