How is heat energy produced

Hello rihanna

Energy can not be produced. Actually : nobody can actually tell you what energy is. We simply do not know. What we call energy is more the power (the work) we can do with different energy formats.

Heat is just one format of energy. And energy can be transformed from one format into another format.
Heat relates to the speed of atoms and molecules. The faster the speed, the hotter they are.
Heat can be produced by burning fossil fuels, or by electric current and resistance, or by nuclear radiation, etc.
Note that all a microwave oven does is exciting the atoms and molecules, which causes them to get hot. Specially water is sensitive to microwave radiation.