I don’t know tons of the detail but I understand the principle. The immutable part of the system is really just an applied oci container image for any ublue based distro.

Certain mount points are writable and persisted (e.g. /home), but otherwise you can just reimage the entire system with any compatible (ublue based) image. Then each image is built by layering changes using ostree. So that’s how you get the different distros.

Silverblue is ublue with gnome, kinoite is ublue with KDE, Bazzite layers steam, proprietary Nvidia drivers and other stuff mainly gaming related, etc.

System updates (which tend to be regular) are just applying an updated image, so actually updating is effectively the same as rebasing.

You can also yourself add ostree layers on top of the base image, and if you rebase to a different one your layers get reapplied on top.