What we know about the Ascended is that the Sun-Disc is not powered by gods, but instead powered by the will of the Emperor of Shurima, as it stated that after Xerath destroyed Azir and the capital, that the Sun-Disc collapsed without the power of the Emperor to upkeep it. What this tells us is that the Sun-Disc most likely draws energy from the sun, but the actual mystical nature of it is connected to the royal family of Shurima or to the will of the people as a whole; Maybe a sort of magical manifestation of the ideas, dreams, hopes, beliefs, and whatnot of the people. This would explain why there is a need for the Ascended to be proven worthy, as only a society would be able to judge who is exalted above others.
Either way, Aurelion Sol stated that both Targon and Shurima were crawling with demigods, meaning that the Ascended and Aspects are on par with one another, though if my theory proves correct, one is powered by extraterrestrial beings and one by the collective wills of the people.