Thank you! More on reddit.com

1,2: Arduino pins will supply whatever current is drawn from them, because nothing is limiting it from inside. You limit current by including resistance in your circuit by adding resistors. If you omit resistors or choose too small value, more current will flow through the IO pin, it will heat up and damage the internal circuitry. The 40mA is per IO pin, and the chip (all IOs together) can supply 200mA at most. More on reddit.com

You can just ignore them unless you need to do something special. Basically an ATMega328P - which is the MCU you are programming on an Uno (and not in your photos) - can't connect directly to USB. So, there is a "thing" in between that can translate Serial to USB. On that board it is the CH340. On a genuine Uno R3 it will be something like an ATMega32u4 (or similar). In both cases there are some pins that are brought out that can be used to configure it or test it. Usually they are just pads on the PCB and don't have pins in them. I'm not sure exactly what those pins are on your, but it looks like TX and RX for two of thm and these would likely by linked to pins 0 and 1 on the headers on the sides of the uno You should ignore them unless you want to do something special that that board is uniquely capable of by offering those pins to you. Also, I think the one you marked as 20, is more likely to be 16 (at least that was the count I got). If I am right then they would be GND, +V, TX and RX (which would make more sense than what they would be if it were a 20 pin CH340 variant). More on reddit.com