It stays shut from cold, then opens to the radiator. It is not the radiator, and not the coolant sensor.
Teaching inline-four schematic. Not a specific car. Coolant thermostat
Shut, coolant stays in the engine so it can warm. Open, flow goes to the radiator. Running temperature is often around 90 °C — not on every engine.
On this schematic: housing at the head outlet, before the top hose.
Temperature needle that never rises (FR article)
Replacement interval is the manufacturer’s. The existing article owns the symptom (needle that never rises), not “what is a thermostat”.