Hey there -
my desktop also had a full screen of files I had not saved on this computer but on a computer I had not used at home for a few years.
When you upgrade macOS, during the Setup Assistant of the new installation, a feature that may be an option to enable is Desktop & Documents in iCloud.
Add your Desktop and Documents files to iCloud Drive - Apple Support
When this is enabled, any files stored in the desktop or documents folder from iCloud will be synced to your Mac. Chances are, your previous Mac had this on, and old files that had been synced are now appearing.
my wife launched the sims4 and had no saved game to load.
You may need to contact the developer of the app to learn more, Contact a third-party vendor - Apple Support, but, if the app saved a local file to your computer, you may be able to use Finder or Spotlight to search for it:
If you can’t find a file on Mac - Apple Support
Additionally, if Desktop & Documents in iCloud is enabled, then disabled, some of your desktop and documents items may be get relocated to a "local" version of those folders found in your home user folder (accessible from Finder > Go > Home).
Hope this helps!
-Jack
