https://en.m.wikipedia.org/wiki/Curie_temperature I doubt they’re using magnets, especially considering how hot they have to get to lose their magnetism as you suggest.

Most thermostats in electronics such as kettles and cookers use a bi-metallic strip inside, where the two metal layers expand at different rates. The contacts in the switch are physically pulled apart by the strip bending when the desired temperature is reached. https://en.m.wikipedia.org/wiki/Bimetallic_strip