iCloud & iCloud Photos · Storage Studio and iCloud
Why can Storage Studio free an iCloud file in one tap but not always a Dropbox or Google Drive file?
Short answer: A macOS rule: Storage Studio may free an iCloud file directly, but for Google Drive, Dropbox and OneDrive macOS limits other apps, so it opens Finder for the provider's own Remove Download. The result is the same — the file stays in the cloud.
It's a macOS rule, not a Storage Studio limit. macOS lets Storage Studio free an iCloud Drive file directly, so Free up in Storage Profile makes it online-only on the spot. For Google Drive, Dropbox and OneDrive, macOS limits what another app can do with those providers' downloaded files, so Storage Studio shows the file in Finder and points you to that provider's own Remove Download.
The result is the same either way: the file stays in the cloud and downloads again when you open it. For Google Drive you can also switch it to Stream files in its own settings so it stops keeping local copies.
See cloud accounts for those providers.