Interjecting with some slight pedantry, but only because I think it’s interesting.

There may be some kettles that just switch off at 100C, but those would be pretty terrible kettles, as they could only boil water at sea level. Go up to 10,000 ft. of elevation, or put something in them that boils at a lower temp than water, and that kettle would just keep running until all the liquid is evaporated.

Most kettles (I think, this is totally based purely on anecdotal evidence, I haven’t actually gone out and examined most kettles) detect the presence of boiling in general, rather than a particular temperature. This allows them to work on a variety of liquids at a variety of pressures (or elevations). They do this with some clever piping and a bi-metallic strip. Basically some of the vapor of whatever liquid you’re boiling is directed through some piping down to the bottom of the kettle, where it passes over a bi-metallic strip and heats it up. Once the strip heats up enough (to a temp much less than the boiling point of water or most other household liquids you find yourself in need of boiling), it buckles, and does electrical circuitry things that end up turning off the heating element.

There’s a Steve Mould video on the topic with a much better explanation that’s super interesting, for those of you into nerdy sciency type stuff: youtu.be/VzqN4Cn8r3U