I would like to know how the PoE on the OrangePi Zero works.
Is it IEEE 802.3af (or IEEE 802.3at) or just "unused pairs powering" ? How to "enable" the PoE function ?
Can someone explain to me ... the use of the " 0 ohm" resistor ?
Yes, the "0 ohm resistor" is simple bridge, which could be easily soldered. Also, it could be used as a jumper.0 ohm resistor could be easily made, but also, you can buy it - they look like usual smd resistor, but with "0" on it.