How does relativity theories explain Entropy?

Answer 1

It doesn't, Goring. Entropy is best explained in statistical mechanics, and there's nothing statistical at all in relativity theory as developed by Einstein. It's a theory based on spacetime manifolds, it's not an "atomic" theory. "Entropy" doesn't appear in any of Einstein's first papers on relativity theory.

When you get into relativistic quantum field theory, then "entropy" makes its appearance, and you have physicists like Stephen Hawkings talking about things like entropy of black holes.