iCloud backup-Documents Folder

As léonie said, they are on your Mac if you don't turn the optimize option on. One way to access them is through the Finder sidebar in the iCloud section. If you want to verify that they are on your Mac, you can open a Finder window and go to your iCloud files, you can hover the cursor over the small cloud icon in the Finder window beside each file or folder and it will show "Downloaded."

My comfort level is high -- but not high enough that I turn on "Optimize Mac Storage." With that option off, all of my files are on both iCloud and also on my Mac where I can personally back them up.

That setting is at Apple menu > System Preferences > Apple ID > iCloud (in Preferences side bar) > Optimize Mac Storage.

Also be aware of the following:

Apple offers iCloud Backup for iPhones and iPads but not for Mac computers.

It is possible to sync data from a Mac to iCloud if you turn on those options (e.g., iCloud Photos, iCloud Drive including the Desktop & Documents option). Syncing has some of the benefits of a backup (and the benefit of being offsite) but is not the same as a backup. Major differences include the fact that not everything on your Mac can be synced and the fact that if you delete something, accidentally or otherwise, there is only a limited time period (30 days in most cases) during which it can be recovered.

Also see: https://support.apple.com/mac-backup

And more information on syncing in:

Set up and use iCloud Photos - Apple Support

and

Access and manage your files in iCloud Drive - Apple Support

and

Add your Desktop and Documents files to iCloud Drive - Apple Support

The files are in [username] > Library > Mobile Documents -- whether or not they have finished uploading to iCloud.

When I did a clone of my entire drive using Carbon Copy Cloner, I was not able to access the files in iCloud Drive like I could other files, due to the way that Finder handles the iCloud Drive folder. I needed to boot from the clone and then I could access them with no issues. Léonie has another method that you may want to consider with Backblaze. Both of these are covered in the following previous forum discussion: https://discussions.apple.com/thread/251398747

As long as you are not using the “optimise mac storage” option, all files you are storing in iCloud will be mirrored locally on your Mac as well and Time Machine will include them in the backup. I do not use backblaze, so I cannot comment on that. Like FoxFifth I am using iCloud for syncing my devices and having the added security of an off-site storage, but I want to have all my data stored locally as well, so I can access them, if the internet is not available, and I want to be able to back them up easily.

Also note that I'm both old school and old.