Connect C wire for nest thermostat on a zoned HVAC system

I got a nest thermostat which requires to get a C wire. My old thermostat does not have c-wire connected, but there are four unconnected wires as shown in the picture.

Then I came to my furnace and found there are two unused wires (orange and blue) near the control board as shown in this picture. The multimeter measures 26 volt between R and Com. First I put the blue wire under Com and measured only 15 volt at the thermostat side between Rc and the blue wire. Then I disconnected the blue wire from the comm terminal and put the orange wire on. I measured 25 volt at the thermostat side between Rc and the orange wire. Why do the two wires give different readings?

Then I realized my HVAC is a zoned system. It has a zone panel. This is the zoning panel diagram.

I also found unconnected orange and blue wires in the zone panel. Using my multimeter, I found the blue/orange wire in the zone panel is connected with blue wire/orange in the HVAC control board. At the bottom of the picture, you can see the wires connected to the thermostat based on my understanding. I don’t see any orange/blue wires there.

So I got totally confused.

1. where is the blue/orange wires behind my thermostat connected to in the HVAC/zone pannel?
2. If I want to have a c-wire, what should I do? Should I connect a wire to the HVAC control board Comm terminal or somewhere on the zone panel board?

Thanks for your suggestion.