can I back up to the iCloud using Catalina?

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

Something else to keep in mind. Unless you have a fiberoptic connection, your upstream speed to Apple is going to be much slower than your downstream. If you are backing up movies, or large photos, or GBs of data, I would say it is better to backup to external hard drive or thumb drive.

150 Mbps = just under 20 MBps. 1024 MB = 1 GB.

As both previous posting have outlined the purpose and functionality of iCloud plus the Speed of Connection issues so very well.

Two cents worth from this corner >>

Do you have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. 

For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways or drive failure

At least one Tested Clone and / or Time Machine Backup should be Off - Site