Plastic!!

The question is too broad...

Cosmetic use for outer casings may well be a plastic like ABS.

Plastic for 'engineering' uses may well be Nylon which may have fillers such as glass for strength.

Where strength & transparency is required polycarbonate is often used

....other plastics are suitable for high temperatures. Google plastic and the specific use you want to know about.

Ah! Some plastic appliances have a recycling mark on the base that looks a bit like the letter delta with a number in the middle. Take your kettle, for example. Have you had a look underneath it? What is moulded into the base?