Upstream images vs bazzite: https://github.com/ublue-os/countme/blob/main/growth_upstream_with_bazzite.svg
My prediction: bazzite may have more installs than Fedora KDE by the end of the year.
Immutable means: every time you power it on, it's the same.
So, if you get infected by a virus, power cycle and it's like it never happened.
That, like everything you read on the internet, is a gross oversimplification, caveats apply, etc. etc. but for the most part, that's what immutable means: the running system has no ability to persist changes (except in very specific, controlled, secure, easier to protect circumstances...)