CrossRoads:
All 5V and GNDs are connected together.
You have up to 800mA of 5V current available for the uC and any peripherals attached, and probably less depending on the voltage at the barrel jack due to the regulator overheating. 500mA if powering from USB.
Can Pins maked with 5V at the corner be used as 5V voltage seperately? This means there is three 5V Supply pin right?
I also do not know what uC means