How does the Arduino RX+GND power supply work?

Question

Arduino

While testing the transmission range over the twisted pair RX-TX, I noticed that the arduino works with the RX cable (0 pin to uno) and ground connected. (without any other food). At the same time, the usual voltages like +3.3 or +5 do not rise on the pins. Hence the question, why does the arduino turn on at all with such a "power supply", and why is it necessary? After all, no sensors can be powered, apparently only the microcontroller itself and the blinking of 13 pins work.

Answer 1

This is a "parasitic" power supply through protective diodes. Read for example here: easyelectronics.ru/parazitnoe-pitanie.html

Answer 2

The controller itself can start working at sufficiently low voltages. Power flows through the protective diodes on the pins - look at the io pin diagram in the datasheet of your controller, it will be clearer.
It's not necessary for some reason.

Answer 3

It is necessary to google "power supply through the protective diode at the input of the IC"