I don't suppose you're familiar with Docker? If you are, you know how you can't really change a container other than through its persistent volumes (otherwise upon a restart all changes are lost)? The immutable images from Fedora as far as I understand are essentially OCI images, so if you think about it from a docker mindset it all starts to make sense very quickly since that is what they practically are.

Or to put it more generally - imagine everything outside of your home folder is mounted as read-only. Flatpaks work great because they're installed in your home folder under ~/.var so you can still install applications this way since your home folder is read/write.

You kinda can install traditional packages using rpm-ostree which creates a layer on top of the system, but it's best to avoid doing this if you can (and keep the amount of layered packages minimal).

This also means that updates are easier to ship out, because the base system everyone runs is going to be the exact same, and untouched. If an update doesn't work for one person, then it probably wouldn't work for anyone running that "image". Additionally, updates end up being atomic in that they either apply correctly or they don't - you won't end up in some weird middle state where your system has part of an update installed.

Fedora's immutable images are indeed the same Fedora system with different DEs. Silverblue is GNOME, Kinoite is KDE, etc. You can actually run one command to "rebase" from one image to another without any data loss too!