iCloud & iCloud Photos · Storage Studio and iCloud
Does Storage Studio upload my files to iCloud?
Short answer: No. iCloud syncing is done by macOS for files you keep in iCloud. Storage Studio only puts a file in iCloud Drive if you move it there yourself; its iCloud features — online-only, duplicates, measuring — run on your Mac.
No. Storage Studio doesn't upload anything to iCloud on its own. iCloud syncing is done by macOS, for the files you've chosen to keep in iCloud Drive or iCloud Photos.
The only time a file reaches iCloud through Storage Studio is when you move it there yourself — for example, choosing iCloud Drive as a destination in Data Parking. Everything else Storage Studio does with iCloud happens on your Mac: making downloaded files online-only, comparing downloaded files for duplicates, and measuring space.
Making a file online-only doesn't upload anything new; the file was already in iCloud. See privacy and safety.