I've just finished the game, and I have doubts about how the post game mechanics work, because this doesn't make sense.
After you finish the game and credits roll, the game proposes you to save and that save is named as "chapter 15...", however when I load that it is a previous save before even fighting Ifrit.
So.. how does that work? where is the game++?. New game is apparently a completely new game, I've tried that just in case.
Some people mention Umbra and going to the past or something, but at that point Umbra is not available, and my main quest is still about going to the Citadel and defeating Ardyn, which I already did.