I'm afraid to say the "Tech" who looked at your PC would appear to be clueless and is giving you a cock and bull story
OneDrive is not a backup, it’s a cloud location of your data.
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The default installation of Office 365 and its OneDrive component uploads all the contents of the PC \My Documents\ folders to OneDrive, (syncs) and there will be a OneDrive folder in File Explorer containing all the OneDrive content. And in Word etc the default Save location is set to OneDrive.
The OneDrive folder in file explorer is a cache copy of the files on OneDrive, delete a file there and it will be deleted on OneDrive
Assuming you are using MS Outlook as a component of MS Office your mail is not stored on OneDrive, its data files are on the local PC and its the same under any win or Office version
In any Office Component > Help > Contact