How to un-iCloud storage of Documents and Desktop

See the following from Add your Desktop and Documents files to iCloud Drive - Apple Support

Turn off Desktop and Documents

1. From your Mac, choose Apple menu  > System Preferences. Click Apple ID, then click iCloud. On macOS Mojave or earlier, choose Apple menu  > System Preferences, then click iCloud.
2. Next to iCloud Drive, click Options.
3. Deselect Desktop & Documents Folders.
4. Click Done.

What happens when you turn off Desktop and Documents

When you turn off Desktop & Documents Folders, your files stay in iCloud Drive and a new Desktop and Documents folder is created on your Mac in the home folder. You can move files from iCloud Drive to your Mac as you need them, or select all of your files and drag them to the place you want to keep them.

Sorry, I don't know why Apple does it that way -- I have some ideas but speculation isn't allowed and Apple rarely publishes reasons.

Turning on Desktop and Documents in iCloud should not have happened without your knowledge even in connection with an update so I can't explain that.

Apple did provide a way to keep your files away from icloud, but the are being as usual a little disingenuous, during your update, you probable say a black screen with a white box with only one blue OK button, this states your files will be moved to the cloud, it appears to be the only way to proceed, this is their attempt at storage expansion in the future if you do not do change now