How does a kettle work and what does the cross section look like?

Answer

I don’t know if I understand your question correctly.

There’s very little in a kettle: there’s a heating element (a resistor, just like on a stove or oven) and a switch that cuts the power as soon as the water boils. Usually that switch is in the handle and works via the steam detection.

See: http://en.wikipedia.org/wiki/Kettle