How do RH/RC/R and Y/W relate for HVAC?
I'll use this gif from my other answer, to try and describe how the system works. I'll focus only on the heating side, just to keep things simple.



When the thermostat is not calling for heat, the circuit is open and electricity cannot flow. Once the temperature in the room drops below the set point, the switch in the thermostat closes. When the switch is closed, electricity can flow through the control circuit.

Electricity flows from one side of the transformer, out to the thermostat on the R wire. It flows through the switch in the thermostat, and back out along the W wire. It then flows through the coil of the heating relay, and back to the other side of the transformer (C). When electricity flows through the relay coil, it moves the switch in the relay from the normally closed (NC) contact, to the normally open (NO) contact. When this happens, the furnace is signaled to turn on.

As you can see in the above animation, when the thermostat contact is open, electricity does not flow in the control circuit.  Once the thermostat closes, the heating relay is pulled closed signalling the furnace to turn on.

At the most basic level, a thermostat is just a switch. So it's not "sending" 24V, it's completing a circuit.