The nature of the Sundering has been spelled out in a Nier Reincarnation side story.

[Lurina]

Since the release of Shadowbringers, and especially over the past several months since the release of Endwalker, there's been some controversy over what exactly happened to people during the Sundering - physically, to their identities, and whether or not this could constitute death. Other than the line from Emet after the Qitana Ravel about people barely remembering the Unsundered World, the game itself is relatively vague.

However, you may remember that there's an ongoing crossover event between Nier Reincarnation and FFXIV. A couple days ago, a new event series focusing on Emet-Selch was released written by Natsuko Ishikawa. In this, the immediate aftermath of the Sundering is directly described. I'll present it here without comment.



If you like, the whole thing can be viewed here. There's some other interesting stuff in there, too. https://www.youtube.com/watch?v=tVZh6oZziwQ