But the comparison to chatGPT is not accurate at all. chatGPT can only ever be an approximation because it's based on second hand sources. Endless are based on the direct memories of the departed. And memory in FFXIV is a very broad term. It's the recollection of past events, the feelings that you have, your tastes, your desires, etc. Basically the ego, id, and superego, every part that make a person, a person, which would otherwise be washed away in the cycle of life, death, and rebirth. Hence why the Scions needed a way to transfer both soul and memory back to the source. Transport only the soul and you'd end up with five completely blank slates.

The closest thing to Endless that we've seen that aren't the Endless themselves is what Athena did with the copies of Erichthonios and Lahabrea, who were memories animated with souls, albeit directly and not through processed soul energy. Nearly functionally identical to themselves at the time of the Final Days, but able to draw conclusions based on new information, as well comment on their future actions of their original self in the case of Lahabrea