When another civilization hits a victory condition (while your own civ is still around) in a single-player game of Civ 6, you get the blowing-dust video that tells you you lost, and can then review various stats for all civilizations in the game. But I can't see any way to find out which civilization won, and with what kind of victory. Is there any way to do so?
According to numerous posts in this thread it looks like this was a design choice and the game will not tell you who won a game when you lose:
Too bad there is no way to tell they reason you lost.
The best suggestion I have found so far was in this thread:
The way I figured it out - hit One More Turn and then go to the victory conditions screen (button on the top right). From there you can go to the "progress towards victory" tab and see who just won and why.