And if we want to be really precise about where energy comes from, it’s worth noting that all elements heavier than hydrogen (i.e. all if them) are the result of stellar fusion. Up to iron in the main phase, and anything heavier in supernovae, neutron star mergers, and possibly other extremely violent events. So fission is extracting the stored energy of dead stars.
Ultimately, it’s probably all just residual energy from the Big Bang.
Other way around. Stars produce energy through nuclear fusion, nuclear reactors produce energy through fission.
And if we want to be really precise about where energy comes from, it’s worth noting that all elements heavier than hydrogen (i.e. all if them) are the result of stellar fusion. Up to iron in the main phase, and anything heavier in supernovae, neutron star mergers, and possibly other extremely violent events. So fission is extracting the stored energy of dead stars.
Ultimately, it’s probably all just residual energy from the Big Bang.