My dryer has a humidity sensor, a thermostat, a thermistor, and I think two fuses. The humidity sensor in the drum has two charged plates that make an electrical contact when wet clothes touch it, which prevents the timer from advancing. As the clothes dry the timer advances more quickly and eventually shuts off the dryer. The thermostat in the heater box cuts on and off to maintain the temperature setting. The thermistor in the exhaust duct and fuses are basically safety devices. I think I got most of that right.
The radiant sensor is a bi-metal warp switch. When heat from the hot surface ignitor warms this switch, it bends to allow a set of contacts to close. This energizes the gas valve coil, which, if it ignites the flame, will sense the heat from the flame, and stay closed. If no flame, then it will cool down and re-try.