iCloud storage space

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.

The closest thing to what I think you are asking for in your post would be to turn on iCloud Drive (with or without turning on the Desktop and Documents option of iCloud Drive) and also turn on Optimize Mac Storage. See the following from: Add your Desktop and Documents files to iCloud Drive - Apple Support

If you need more space on your device, iCloud Drive can help. On your Mac, choose Apple menu  > System Preferences. Click Apple ID, then click iCloud. On macOS Mojave or earlier, choose Apple menu  > System Preferences, then click iCloud. Turn on Optimize Mac Storage. Then your Mac keeps all of your recent files on your computer, but keeps your older ones only in iCloud, ready for you to download when you need them again. 

Other information in:

How to back up your Mac - Official Apple Support

And more information on syncing in:

Set up and use iCloud Photos - Apple Support

and

iCloud Drive FAQ - Apple Support

iCloud is greatly misunderstood. Except for mobile devices it isn't backup space. Nor is it a hard drive in the sky for storage except in a limited capacity. It is really syncing storage. You can use it for storage of some things if you "optimize" a device or computer. That is described in these articles:

How to free up storage space on your Mac - Apple Support

Optimized Storage [macOS Sierra or later] helps you save storage space space by storing your content in iCloud and making it available on demand:

- When storage space is needed, files, photos, movies, email attachments, and other files that you seldom use are stored in iCloud automatically.

- Each file stays right where you last saved it, and downloads when you open it.

- Files that you’ve used recently remain on your Mac, along with optimized versions of your photos.

How to check the storage on your iPhone, iPad, and iPod touch - https://support.apple.com/HT201656 - "If your device is low on storage, iOS automatically optimizes its available space while installing an app, updating iOS, downloading music, recording videos, and more. To make more storage available, iOS can remove some of your items, like streamed music and videos, files in iCloud Drive, and parts of apps that aren't needed. It also removes temporary files and clears the cache on your device. iOS only removes items that can be downloaded again or that aren't needed anymore."