It is explained in the support article that was linked. Please read it.
@ChaucerGeorge When you unlink OneDrive or prevent it from syncing data, the files and folders OneDrive was syncing are removed from your computer, leaving only the copy in cloud storage. There are two ways around this. One is to stop the syncing without unlinking OneDrive, download the files you want to keep, and then unlink if you want to. (There's no harm in letting OneDrive keep running, but maybe you want it out of the way.)
The other approach is to make a backup copy of your Sims 4 folder, or whatever you're looking to preserve, before making any changes. You can place the backup on an external drive or USB stick, or simply drop it into a folder that OneDrive doesn't touch, for example Downloads. Either way, once OneDrive has removed the original copy from your Documents directory, you can restore the backup.
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