As a happy user of Joplin Cloud for the past year, I'm surprised I don't know the answer to this question and I didn't see it clearly answered on the site or in the forums: where is my data kept when I sync with Joplin Cloud?
For example, if I lose all my devices, can I recreate my Joplin notebooks merely by firing up a new Joplin instance and connecting to my Cloud account?
Another way to ask this is: could a hacker, or a government with a subpoena, get access to my notebooks if they had access to the Cloud server?
As an aside, whilst what @laurent says is perfectly true you must remember that the data on JoplinCloud is not a backup. It is only the method used for your note data to be synced from client to client. If something odd happens and your client is emptied of your notes, that could get synced upwards and onwards. Basically if you also want the safety of having a backup (and if you have not already done so) you should consider using something like the Simple Backup Plugin to keep a local rolling backup archive of your note data for disaster recovery.