What happens in the Arduino power supply schematic?
1) It is a comparator. It is switching the MOSFET on and off depending whether or not the Vin is present in order to choose to power the board from Vin or from USB.

2) It is a buffer separating between the SCK pin and the LED. The LED is intended to blink while the board is being programmed.

3) It is converted to have 3.3V supply on the board in order to provide it to potential sensors/shields requiring it (and as the reference to the comparator above, but it is not necessary as it can be taken out of voltage divider).