Yeah, flatpak has some good ideas, and they're even mostly well executed, but once you start trying to build your own flatpaks or look under the hood there's a lot of "magic". (Examples: Where do runtimes come from? I couldn't find any docs other than a note that says to not worry about it because you should never ever try to make your own, and I couldn't even figure out the git repos that appear to create the official ones. How do you build software? Well, mostly you plug it into the existing buildsystems and hope that works, though I mostly resorted to `buildsystem: simple` and doing it by hand.) For bonus points, I'm pretty sure 1. flatpaks are actually pretty conceptually simple; the whole base is in /usr and the whole app is in /app and that's it, and 2. the whole thing could have been a thin wrapper over docker/podman like x11docker taken in a slightly different direction.

Well flatpak was started pre oci. But its core is is just ostree + bwrap. Bwrap does the sandboxing and ostree handles the storage and mount. Now there still a few more stuff but these 2 are the equivalent to docker. Bwrap is also used for steam and some other sandboxing usecases. Ostree is the core of fedora silverblue. Runtimes are special distros in a way, but since the official one are pretty building everything from source so the repos tend to be messy with buildscripts for everything.

Not sure what you're talking about, Flatpak runtimes are easy to find and contribute to: https://docs.flatpak.org/en/latest/available-runtimes.html

I wasn't directly involved, but the company I worked for has created its own set of runtimes too and I haven't heard any excessive complaints on internal chats, so I don't think it's as arcane as you make it sound either.

You can build your own flatpak by wrapping bwrap, because that is what Flatpak does. Flatpak seems to have some "convenience things" like the various *-SDK packages, but I don't know how much convenience that provides.

The flatpak ecosystem is problematic in that most packages are granted too much rights by default.