Security in clouds while files/folders open in IOS - vulnerable?

Pardon my lack of tech knowledge when it comes to encryption and security.
Question: What happen to an encrypted file in the cloud (Dropbox or iCloud for example) once you open it with the Cryptomator app on a smartphone? Is it vulnerable once it is opened inside the cloud therefore making vulnerable if your cloud account was hacked or if a technician at the cloud wants to snoop in and look at your files? is my file remains encrypted in the cloud while only visible in the Cryptomator app while it is opened on my iphone…? Do the files remain encrypted for the entire time on the cloud side while opened in the App?.
Hope my question is not too confusing.

Nothing, it stays encrypted. You download the encrypted file and decrypt it locally in the app to view its content.

No, since you don’t open it in the cloud.

Yes. The app just provides a decrypted view of your vault, so that you can browse it.