what makes our body warm?
In summary, cellular respiration and thermogenin.

Most of the energy in our body comes from cellular respiration. Cellular respiration is how ATP, the major source of cellular energy, is made from glucose. Cellular respiration generates heat mo matter what because not all of the energy from glucose is converted to ATP.

In certain cells, a protein known as thermogenin allows the protons in the cellular respiration to flow from the inner membrane to the matrix (the protons are in the inner membrane of the mitochondria as the electrons from glucose were used to pump them). This generates heat. In regular cellular respiration, the proton gradient is used to make ATP, but now, it juet generates heat.

Also, ATP can be hydrolyzed directly in brown fat cells and muscle cells to produce heat along with inorganic phosphate and ADP.