I have elderly parents who are easily confused, so I feel your pain.

In this case, your parents aren't to blame -- This was caused by OneDrive. Certain Windows Updates will cause OneDrive to have its settings put back to defaults. The Backup option does the behavior you mentioned, moves everything to the cloud, remaps the user's folders to OneDrive folders, deletes local copies, etc.

FIRST and foremost -- make sure you have a second copy somewhere OneDrive isn't looking before you start anything. Some people found out the hard way that once you delete OneDrive, there go all of their data they thought was stored locally.

I like Chris Titus' Windows Tool to do a lot of things. https://christitus.com/windows-tool/

Under Tweaks is Uninstall OneDrive. I like his approach because it makes sure everything has been downloaded from OneDrive before removing it.

Also under Updates, he allows you to disable updates to various degrees. The Security setting is quite a nice compromise. No new "feature" updates for a long time, and it waits 4 days for security updates, so it lets the other users be the guinea pigs.