iCloud & iCloud Photos · Storage Studio and iCloud
Do I need to add iCloud as a cloud account in Storage Studio?
Short answer: No. Storage Studio never asks for your Apple Account password. It works with the iCloud Drive folder macOS already keeps on your Mac, so once you're signed in to iCloud in System Settings, Storage Studio finds it automatically.
No. You never sign in to iCloud inside Storage Studio, and it never asks for your Apple Account password. iCloud Drive is part of macOS, so Storage Studio simply works with the iCloud Drive folder that's already on your Mac once you're signed in to iCloud in System Settings.
The accounts you add in Remotes & Accounts are separate services you connect yourself, such as Google Drive or an S3 bucket. iCloud doesn't appear in that list because it doesn't need to.
If iCloud Drive is turned on in System Settings → your name → iCloud, Storage Studio finds it automatically — it shows up in Optimize Cloud Storage, in the Duplicates window and in Storage Profile. See what Storage Studio does with iCloud Drive.