AlxDroidDev:
The schematics won't show the other pins for the LM358. What powers the IC? Since power source hasn't been selected yet, where does it get its power (5V, I guess) from?
Well it's called a auto-voltage selection circuit, but in reality it operates as a auto voltage cut-off circuit. If there is external power connected via the connector (or Vin pin) of at least the minimum value then that hardwires to the on-board voltage regulator which then generates +5vdc Vcc power which is hardwired to the board's Vcc bus. The op-amp comparator seeing this valid Vin voltage then switches it's output to shut-off the mosfet switch that in turns prevents the USB's +5vdc source from being routed to the board's Vcc buss. That make sense?
Lefty