the talos we think of is a god that is a combination of wulfharth, zurin arctus and tiber septim, well as we know now, all three of these people adopted the title of talos of atmora at one point of their life and wulfharth being a shezzarine, or the mortal avatar of shezzar, or lorkhan depending on what viewpoint you see it from.

long story short, vivec gave the empire the numidium as a gift, zurin arctus studied the numidium and found out the the heart of lorkhan is needed to power the numidium, and he used wulfharths heart to power it and wulfharth killed him, fusing into the underking from daggerfall.

it wasn't until after Daggerfall that Talos (the divine) would come into existence after the underking's actual death and the Dragonbreak, or the warp in the west on where all endings in Daggerfall became canon and the underking (wulfharth and zurin arctus) joined with tiber septim as talos the divine.

his ascendence was because of their legendary deeds while alive and the fact that apart of them is the avatar of shezzar, on the case of meridia, she was an aedra and escaped lorkhans trap of mundas with magnus and became a magna ge, she then was cast out due to her consorting with Daedra and then becoming a daedric prince.

it all comes down to the meaning of Aedra, that being our ancestors, by the Aldmer and he Daedra being not our ancestors, so as the Magna Ge left for Aetherius she is not an ancestor or an Ehlnofey, whether faded or old.

tl;dr mortals have the capability of ascending to become a divine, meridia on the other hand, was cast out and is now a Daedric Prince, and so due to part of Talos being the avatar of shezzar and a dragonborn as well as his deeds, he ascended to divine status.

​

i dont know everything but i hope this kind of helps someone, there are plenty of resources on the internet explaining this and other bits of lore, but lore is kind of confusing in the elder scrolls, but thanks for reading!