The easiest way to think of it is flatpaks are AppImages with a repository and snaps are flatpaks but bad.

That has benefits and detriments. Appimages contain everything they need to run, flatpak’s mostly do, but can also use runtimes that are shared between flatpaks.

All flatpaks are sandboxed, which tends to make them more secure. AppImages can be sandboxed, but many aren’t.

Flatpaks tend to integrate with the host system better, you can (kinda) theme them, their updates are handled via the flatpak repo, and they register apps with the system.

AppImages are infinitely more portable. Everything’s in one file, so you can pretty much just copy that to any system and you have the app.

Yeah, but people installing GrapheneOS probably aren’t in the vast majority of users