/home/<user>/.local/share/Colossal Order/Saves
real handy tip if you dual boot
enter the following command in your terminal:
ln -ls /mnt/c/Users/AppData/Colossal\ Order/Cities_Skylines/Saves ~/.local/share/Colossal\ Order/Cities_Skylines/Saves
replace /mnt/c with the actual path to your NTFS partition where your Users folder is located
and now your saves automatically transfer between boots
What this does is it creates a symbolic link, which is basically a shortcut except your programs can't tell the difference, which directs it automatically to the other drive.
Hope this helps!