Your data is stored locally on your phone. You can optionally store the backup on your private Google Drive space or iCloud.
Data stored in the app's private directories are not accessible by any other apps or processes. You (or anyone else) even can't download the data from the app even if you connect your phone to your computer.
If you enable backups, they are transferred to Google Drive or iCloud via the secure (encrypted) channels. All data stored on Google Drive is encrypted by Google, data on iCloud is encrypted by Apple.
The most important thing is that only Nutrilio is able to read your entries. We don't send your data to our servers so we don't have the access to your entries. Also, any other third-party app can't read your data.