How is current controlled in high power applications?
simple electric ovens use simple temperature control usually using something like a bimetallic strip (you can here them click in and out) where the dial is used to set the point of electrical contact that will then provide electrical power to the heating elements.

the result is more of a hysteric control with a very long duty cycle (where the duty is governed by the thermal characteristics of the oven - ie how long to cool) and the lower band to-do with specific characteristics of the thermistor/bimetalic strip.

More modern electronic controllers do a similar thing but use thermalcouples/RTD to measure the  temp and then semiconductors to switch the power (SCR's)