Arduino power input, what pin can you use for 12V DC to supply my Arduino? What's voltage limit can the Arduino can handle?
There is a linear regulator such as LM1117 on the Arduino (and clone) boards you mention. The MCU chip itself does not draw much current, but if you draw current from the GPIO pins or connect something that draws a lot of current to the 5V pin the regulator will get very hot if the input voltage is too high.
So, while you can use 12V (and the LM1117 can withstand a bit more) it's better to keep the input voltage in the 7-9V range in many cases. The LM1117 drops out at a bit over 1V worst case, so 6.5 is about the minimum you should use. The official page (for the Nano) is not self-consistent with one page showing 6-20V and the Tech Specs showing a more sensible 7-12V.