desktop and documents gone in iCloud?

Hello,

After an unsuccessful upgrade on my older iMac to Catalina, I decided to wipe the drive and bring her back to High Sierra. I haven't back her up in years, to answer your first question. I purchased a new 4TB external drive and copied the files over while I was booting off of another external drive running Sierra that used to be in my MacBook. I noticed there was a 'blank' Macintosh HD on the iMac and that was the first to get deleted. Probably a mistake. As I was double checking to see if I missed anything I noticed my Desktop and Documents folder were gone. I assumed they were in iCloud Drive and all would be good. Except they weren't. It looks like they were replaced by the desktop and documents of my external drive I was booting off of. I tied to reboot into Catalina but now that doesn't work so I'm assuming deleting xtra HD image broke that. When I reinstall High Sierra on the iMac will my files reappear in iCloud?

If not, what the point of the iCloud Drive if your HD STB? Thanks for any help.