C wire - Where does it go?
Fortunately, in your case, the fat blue wires in your first photo are the transformer return -- connect the skinny blue wire from the thermostat cable to the fat blue wires. Since the air-handler maker used quick-connect terminals for the internal wiring, you can crimp one of these onto the end of the thin blue wire and slide it on in place of one of the fat blue wires somewhere, then slide the fat blue wire onto the stacked end the linked terminal provides.