iCloud & iCloud Photos · Storage Studio and iCloud
Does Storage Studio scan iCloud Drive for duplicates on its own?
Short answer: Yes, if Auto-scan is on (the default) for the iCloud Drive source in the Duplicates window. Scans of your Mac's files wait for idle time, only downloaded iCloud files are compared, and Scan now runs one on demand.
Yes, if you leave it on. Each source in the Duplicates window — including iCloud Drive — has its own Auto-scan switch, on by default, so results are ready when you open the window. Scans of your Mac's own files wait until it's idle so they don't compete with your work.
You can also scan on demand with Scan now (it turns into Stop while it runs), and it only scans the source you're looking at.
Only iCloud files already downloaded to your Mac are compared; online-only files are skipped, so a scan never downloads anything. Results are remembered between launches.