TL;DR: Your data remains private, on your phone, where it belongs.
Your data stays on your phone and/or watch, in the Steadfast app, at all times.
Steadfast does not access your data, other than to display it on your phone and watch. Your data is not analyzed, transformed or transmitted to any service, and Steadfast makes no use of third party logging or analytics libraries which could potentially leak data. The maker of Steadfast has no access to your data.
Where logs are recorded to the device for debugging purposes, any personal data is obfuscated.
Your data will be backed up during iCloud backups, and will be restored if you restore from such a backup. However, if you delete and re-download Steadfast, your data will be lost, as Steadfast does not sync via any cloud service.
Steadfast does not track use or users, either individually or in aggregate.