assistant is a private service run by Hannes Uebelacker for himself only. It has no other users.
Documents Hannes files in it, and the access granted to it for his own Google account (Gmail and Google Calendar). Everything is stored encrypted on Hannes' own server.
Only Hannes. The data is never shared, sold or handed to anyone, and it is used for nothing but Hannes' own mail, calendar and documents. The service never sends mail and never invites anyone to an event; it only writes drafts and Hannes' own calendar entries.
Access to the Google account can be revoked at any time in the Google account under Security, Third-party apps with account access.