Why is Fu Hua immortal?

I'm intrigued by the claim that Fu Hua is immortal. Can you elaborate on the reasons behind this assertion? Is there a particular myth or legend surrounding Fu Hua that explains their longevity? Or perhaps there are scientific explanations, such as advanced medical treatments or genetic modifications, that have contributed to their seemingly endless lifespan? I'm eager to learn more about the unique qualities and circumstances that have led to Fu Hua being labeled as immortal.