I think about it like this:
Layer 2b: ->> User applications (flatpak, nixpkgs, etc.)
Layer 2a: ->> User data (mutable, persistent no matter what your system layer is)
Layer 1: -> System (immutable/read-only/updated "atomically" meaning all at once)
Layer 0: Hardware
Or, alternately, it’s what macos has been doing with absolutely no fanfare for several versions now. That’s not a knock, btw. It’s an illustration that it can be completely transparent in use, though it may require some habit changes on linux.
It seems to still be strongly gnome-adjacent, which fits with the softer, “calmer” aesthetic Pop has, but with functional tweaks that are more aligned with Win11/KDE (absolutely intended as a positive statement, as far as moving the ball forward on UX design). I worry that team KDE won’t like the “sane defaults” simplicity that it appears to have inherited from the gnome days, but that might just be the part of me that experiences terminal choice paralysis every time I fire up KDE. :)