Elements (editor or organizer) can't access and work directly on files 'on the Cloud'.
The nearest solution would be to store your photo files in a dedicated folder installed by Cloud sharing services like OneDrive, DropBox or Google Drive. Those services install those dedicated folders and manage the 'synching' to the Cloud. The organizer or the editor see the files on those folders just as other normal folders. The photo files copies on the Cloud are accessible from other devices or computers.
- The catalog itself (the database) only knows the files on the dedicated folders.
- Another computer accessing the files on the Cloud can see the files, but can't access the catalog.
- The catalog folder stays on your computer, either in the default location or in a 'custom location'.
- You can't use that synching solution to save space on your computer: there is always a duplication from computer to Cloud and vice versa.
- The current PSE2021 has an option to backup the catalog structure, not the photo files, on the Adobe 'Creative Cloud'. That does not answer your question, but it may be very useful to recover the catalog structure after a corruption.