You can actually build “from source/locally” with flatpak. And its actually pretty simple, because in most cases you should only really need the manifest.json/yaml of the flatpak application (contains all the build instructions), and with it you can build the application locally with the flatpak-builder command. (ref. https://docs.flatpak.org/en/latest/flatpak-builder.html )

This seemed like something people might misunderstand when they view the strip, so i thought it might be interesting to add this information here.