C wire for a smart thermostat
Somebody was thoughtful
The good news is that somebody was thoughtful and ran a thermostat cable with extra wires in it, so you have the spare wire you now need.  So, take that blue wire, land it on the C terminal on your furnace board alongside the existing C connection from your air conditioner (if you need to extend it, just get some blue 18AWG TFFN and nut it to the existing blue wire, then land this pigtail on the C terminal screw, being careful not to undo the existing connection of course), hook the blue wire up to the C terminal on your new thermostat, turn the furnace back on, and enjoy!