Signal only provides a script for .deb based distros on their official website. The flatpak is currently not ideal because it stores encryption keys in plaintext.
The provided link suggests an automated installation in a Ubuntu Distrobox including automated updates. Useful for every distro that does not natively support .deb packages.
All good. Checkout the flatpak called “gear leaver” for managing and updating appimages. It’s really helpful. If your using a ublue image it might already be installed.