iCloud back up of files not associated with an Apple application

Macs back up using Time Machine (or similar).

Back up your Mac with Time Machine - Apple Support

As muguy implies, iCloud is not backup really. It is the ability to share files with other Apple devices.

Depending on how you have iCloud configured, it might be the ONLY copy of a file, and you get access by first downloading it to your Mac. That is a situation where there is ONLY 1 copy of your file and it is in Apple's hands, which have humans working for them, and sometimes humans make mistakes. an Oops email is not what you want to receive.

However, you can put just about any file you want into your iCloud Drive.

Finder -> Settings -> Sidebar (tab) -> [X] iCloud Drive

In any Finder sidebar should now be an entry for "iCloud Drive"

You can copy any files you want into that folder and they will be sync'ed to iCloud and also made available to all your other Apple devices (iPhone, other Macs, iPad, ...)

If you are familiar with Dropbox, this is similar in that you have a folder which is sync'ed with iCloud and back down to other devices, or at least accessible to other devices via their iCloud Drive access.

But you should really consider having a real backup, such as Time Machine to local storage. Just about any USB external disk drive will work as a Time Machine backup device. If you are up-for-it, then maybe even a Synology NAS as your Time Machine destination (more cost and more complexity, but you get to backup over the network to a system somewhere in your home).