The cheap cookers use a mechanical setup as there are no electronics. I have the cheapest Zojirushi 3-cup cooker where you just push down a lever. That lever mechanically latches a spring loaded mechanism that closes the heating element switch. The temperature will stay relatively stable as the water absorbs the heat keeping it at around 100 C. Once the water is evaporated from the bottom the temperature rises which trips a bimetallic mechanism releasing the spring loaded lever opening the heater switch. Stupid simple.