Hidden Library Files Backed up to iCloud?

This won't help for your current issue, but Quicken allows you to change where the Quicken data files and backups are stored. Mine are stored in my Documents folder and I have iCloud Drive with the Desktop and Documents option turned on, so my Quicken data files and Quicken automatic backups are synced with iCloud.

No. A Mac will only sync files in the iCloud Drive folder (including Desktop & Documents folders if that option is turned on).

Also not that 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 the Apple article: How to back up your Mac - Official Apple Support

And more information on syncing in:

Set up and use iCloud Photos - Apple Support

and

Set up iCloud Drive on all your devices - Apple Support

and

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

You mentioned - and my MAC crashed.

Erase the hard drive and reinstall this affected Mac Erase and reinstall macOS - Apple Support

also - Are the hidden Library files that support applications on a MAC backed up to the iCloud?

No .

Once you reinstall Mac , sign in with same Apple ID and password on the same network between two Mac , turn on iCloud Drive , the user library files are never saved in iCloud they are confined to each Mac , don't delete any files from user library or drag and drop to a folder that is saved in iCloud Drive .