iCloud Drive is a syncing service. If you delete a document from an iCloud Drive synced folder in Finder on your Mac it will also delete from iCloud. If your delete it from www.iclolud.com it will also delete from any synced devices.
If you want to reduce storage on iCloud, one way to do that would be to move some files from an iCloud Drive synced folder on your Mac to a folder that is not synced with iCloud Drive. You can do that using Finder on your Mac.
Or iff you have the Desktop and Documents option of iCloud Drive turned on and want to discontinue syncing of those folders, you can turn that option off as described in the following excerpt from Add your Desktop and Documents files to iCloud Drive - Apple Support
Turn off Desktop and Documents
- From your Mac, choose Apple menu > System Settings.
- Click Apple Account. In macOS Sonoma or earlier, click Apple ID.
- Click iCloud.
- Under Saved to iCloud, click Drive. In macOS Sonoma or earlier, under Apps Using iCloud, click iCloud Drive.
- Turn off Desktop & Documents Folders.
- Click Done.
What happens when you turn off Desktop and Documents
When you turn off Desktop & Documents Folders, your files stay in iCloud Drive and a new Desktop and Documents folder is created on your Mac in the home folder. You can move files from iCloud Drive to your Mac as you need them, or select all of your files and drag them to the place you want to keep them.
or if you want to completely turn off iCloud Drive on your Mac, see the following from that same support article:
What happens when you turn off iCloud Drive or sign out of iCloud
If you turn off iCloud Drive or sign out of iCloud, a new Desktop and Documents folder is created in your home folder. You also have the option to keep a local copy of your files that are in iCloud Drive. If you keep a local copy of your files in iCloud Drive, they're copied to a folder called iCloud Drive (Archive) in your home folder. You can move any files that were in your iCloud Desktop and Documents to your new local Desktop and Documents.