If you already have the correct version of the flatpak installed, you can try flatpak build-bundle.

flatpak build-bundle LOCATION FILENAME NAME where

• LOCATION is the path of the repo on disk. Run flatpak info -l org.kde.arianna, and copy the part before /app
• FILENAME is the output file name, preferably .flatpak. Eg: arianna.flatpak
• NAME is the name of the app, here org.kde.arianna

The generated file can be installed with a double-click, or with flatpak install <file>

This is the equivalent of an Android .apk. It contains the app but depends on a runtime. If you want to install it in a few years, odds are the runtime will no longer be available. You can backup the runtime the same way with the --runtime option.

flatpak build-bundle --runtime LOCATION FILENAME NAME where

• LOCATION same as earlier
• FILENAME eg arianna-runtime.flatpak
• NAME is the name of the runtime, which you can get with flatpak info --show-runtime org.kde.arianna

This takes a while, for some reason. Maybe it’s compressing stuff?

The runtime is installed the same way as the app: double click or flatpak install.

Note: I only did this once, and not specifically on Arianna. Hope it works.