It is not possible to open Notion files from iCloud Drive on an iOS device. Notion for iOS, will create it's own folder in iCloud drive. If you are on a desktop, anything you drop in that folder will show up on the Home screen of the Notion for iOS app. Any files that you create in the Notion for iOS app will in turn show up in the Notion folder in iCloud Drive on a desktop. This is the best way to sync with Notion. If you need to enable iCloud support for Notion for iOS, simply go into your Device Settings and scroll down to the Notion app.