I just read through the unofficial Flathub Flatpak for Signal and it is very simple. It fetches the .deb from Signal’s website, installs it in the sandbox, and uses a launcher script to tell the OS some basic toggles like should it start minimized or should it display a tray icon. In the script it makes use of zypak, which to my understanding is to tell electron (chromium) to allow sandboxing to be handled by Flatpak. Here is the repo and the build instructions is the .yaml file.

Flatpaks are pretty easy to read through. Just go to the links section of Flathub and click the manifest, then read it to see what is done during building.

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Just because something is built out of love does not make it safe, and attestation is about safety. You wouldn’t trust an un-attested surgical device, just because there’s a really positive community around its design.

Signal is a life-or-death app for some people.

Well I think you have to distinguish between a messenger and other programms, because a messenger has a lot of sensitive data.