What a thermostat actually does
Crack a thermostat housing before the engine is warm and the disc is sitting on its seat. Coolant is trapped in the block. The radiator is just a tank on the nose. The thing holding that door closed is a wax pellet in a brass cup. That is the thermostat. It is not the needle on the dash.
The pellet is the whole trick. Cold, the wax is solid and a spring keeps the disc shut. Heat soaks the cup. The wax melts, swells, and a pin shoves the disc off the seat. Coolant can finally walk to the radiator and dump heat. The gauge is a sender in the neck or the head. It only reports. This valve decides whether the loop is open.
That is why you want it closed for the first few minutes. Open too soon and the engine never gets to temp, the heater is a suggestion, and the computer stays in a warm-up map. Stuck shut and the radiator never sees the heat. The needle climbs. The upper hose gets hard. People flush the radiator. The door never opened.
Pull the hose and look at the disc. A live one sits flat in the housing. A dead one is cocked, stuck, or the wax has cooked into a lump that never moves. The sender in the neck is the gauge. The thermostat is the door.