You can save an editable copy of your typefaces (including strokes & background data), for backup or to transfer on another device.
Creating an archive
To backup a specific font
From the Home Screen, press a long tap on any font and tap Archive
Save the archive file under the desired filename & folder location
You can also tap Save to Files to pick a precise location on your device
To backup all fonts
From the Home Screen, tap the help icon and tap Archive All Fonts
Save the archive file under the desired filename & folder location
Restoring an archive
Launch the Files app
Locate your font archive—it's a file with the extension
.fslf
Select the archive and tap Share
Pick the Fontself app icon
Et voilà: each font will be imported as a new copy to avoid overwriting any previous version.