Heat generation through human body

By the fact all human combined output about 1.3 terawatts of enery .
So from where this amount of energy comes from
Lets check it out the reason below:-

Humans are big sacks of metabolic chemical soup, there's always something going on.

Most of these chemical reactions release heat as a byproduct

The most basic form of energy our body uses to power everything we do is Adenosine Triphosphate (ATP):

ATP does everything: from making your muscles contract to powering ion pumps that make you think. Your body makes ATP work by shearing off the final phosphate group, releasing a lot of energy. About 60% of the energy released from this reaction is wasted heat.[1]

The creation of ATP creates waste energy also, the krebs cycle, where sugars are oxidized to ATP and beta oxidation, where fats are oxidized to sugars all release heat.  Mitochondria is the key to your body's heat production as most of the ATP creation and fuel oxidation occurs in these little powerhouses.

Mitochondria

When you're cold, your hypothalmussignals your thyroid to send out signals to increase your metabolic processes: makes you shiver and tells your cells to make more energy.

Your body's processes revolve around creating energy and managing temperatur