Backup
One file. Sealed with a key only you know.
Your backup is a single file, sealed with a key of your own. A key can be a whole sentence. You decide where the file goes.
Making the file
In noapps, open Menu, then Settings, then Backup. Choose a key and make the file. noapps sends it nowhere; you decide where it goes.
What is in it
Everything you keep in noapps, Letters included, sealed with your key. The file also holds a reading page that opens offline in any browser: type your key and read what it opens.
A new phone
Move the file to the new phone, for example with Quick Share or AirDrop, the Files app or a cable. Install noapps, tap "Restore from a backup" on the last page of the start, choose the file and type your key. Everything comes back, Letters included, without scanning anyone again.
The iPhone backup
On iPhone you can also include noapps in the iPhone backup. It is off until you turn it on. On: all of noapps, Letters included, becomes part of your iPhone's own backup to iCloud or to a computer. Apple handles that backup under Apple's terms. Guarded layers stay sealed by their key either way.
Keep the file, keep the key
Keep the file somewhere safe, and keep the key somewhere other than the file. Without the key, nobody opens the file.
A lost key cannot be recovered by anyone. Including us. If you lose your phone and have no backup file, nothing can be restored either: nothing ever reaches us, so there is nothing we could restore.