can I copy my desktop mac files to iCloud, then modify them from a laptop?

Of course. Enable iCloud Drive on your Mac(s) (you can also enable Desktop and Documents). System Preferences > Apple ID > iCloud. (you can also choose to optimize files on your Mac)

And on your phone (Settings > Apple ID > iCloud Drive)

Learn more: https://support.apple.com/en-us/HT201104

Arre Aye Vee Said:

"can I copy my desktop mac files to iCloud, then modify them from a laptop? how to keep files updated across iMac, laptop 15" and iPhone?"

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Try iCloud Drive, Directly:

See if you can upload them from your from computer here: https://www.icloud.com/iclouddrive