All of the app data is stored on your local device or your own personal Apple iCloud account. It is not shared with the developer or available to anyone else. Only those that have access to your devices have access to your data.
See below for additional information regarding data storage for particular apps.
All the contact related information (name, address, phone numbers, emails, contact photo etc) in Clever Contacts is actually saved in and read from your Contacts App on the device. Membership in groups (e.g. Family), notes, photos associated with notes and other settings data is maintained by Clever Contacts in separate files. These will either be only on your local device or in your iCloud account depending on which storage option you have chosen in the Clever Contacts “Settings” screen.