Red is +24VAC. RH is the heating side. RC is the cooling side. Typically, you only use the RH.
W1 is typically “Furnace”. So that is why it calls for heat. W1 and W2 are used if you have a dual-stage furnace.
With most smart thermostats, you also need a Blue C wire, which is -24VAC, to power the thermostat. Or, the thermostat runs on batteries.
Y1 and Y2 are Yellow and those are used for A/C, two being for if you have dual stage.
G is Green for fan.
Hope that helps.
But, most code will not allow you to turn on or off fireplaces unless someone is physically there. Keep that in mind.