How is the law of conservation of energy manifested in thermonuclear fusion?

Question

Physics

I ask you to direct your thoughts in the right direction, Google and reading did not help to find the answer to the very essence: where does energy come from during synthesis? In simple terms, on the fingers.
Gasoline burns - this is the energy that was previously spent on the creation of chem. connections.
Uranium decays - it is understandable, energy is stored in the heaviest atom, the mass defect passes into the kinetic energy of the fragments.
Hydrogen isotopes collapse into helium, and a lot of energy to boot - how? Where? Who spent this energy and where to release it here?
Thank you.

Answer 1

Again, the mass defect, the helium core is slightly lighter than its constituents. Light atoms tend to merge, heavy atoms tend to disintegrate.
Iron is closer to the middle, its decay or fusion is unfavorable, one of the most stable and therefore common elements
www.fizika.ru/kniga/tema-15/p-15e-4.gif