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Not a forum issue, so moved to the right forum.
 
If your cities are saved to the Steam cloud they will still be there afterwards.

If however they are locally saved, they will be deleted along with everything else. They are written to:
...\AppData\Local\Colossal Order\Cities Skylines\saves\
(To see the AppData folder you'll need to enable View Hidden Items in File Explorer)

So you should copy them to a USB stick or somewhere else safe first.
 
If your cities are saved to the Steam cloud they will still be there afterwards.

If however they are locally saved, they will be deleted along with everything else. They are written to:
...\AppData\Local\Colossal Order\Cities Skylines\saves\
(To see the AppData folder you'll need to enable View Hidden Items in File Explorer)

So you should copy them to a USB stick or somewhere else safe first.
And if I downloaded the game from Xbox Game Pass
 
And if I downloaded the game from Xbox Game Pass
The same applies - if you chose Cloud saves they are in the MS cloud and will be unaffected by what you do to your PC.

If they are local, they are in the same place I just gave you.