Nest 2 wire hookup:
Some “smart” thermostats are not really all that smart and require a third “common” wire so that continuous power is supplied to the thermostat even when there is a call for heat.
For only 2 wires Nest has a rechargeable battery that charges when there in no call for heat using the 24V open circuit voltage. When there is a call for heat the 2 wires are “short circuited” sending the 24V to the boiler or furnace control module. While there is a call for the heat the Nest runs on battery. When the call for heat ends, back to recharging. Pretty simple.
Lots of video help on hookup:
https://www.onehoursmarthome.com/blo...-2-wire-hookup
Our 30-year-old home has 3 zones all with 2 wire thermostats. We recently had a modern boiler installed that was setup for 2 wire thermostats. Rewiring the thermostats for 3 wire would not be easy as in most homes. So it makes sense that Nest has a 2 wire solution.