What do you mean by immutable? Do you mean point release? Why would anyone use a point release distro for the desktop is beyond me.
No, he means Immutable distros like Fedora Silverblue. https://fedoraproject.org/silverblue/
Baisically the base distro is read-only (thus immutable) and all changes are made in top of that.
Makes it hard to bork a system and difficult for attacks to affect you.
No, I am using fedora silverblue which is point release. But there are rolling release immutable distros like opensuse aeon/kalpa im pretty sure. Basically the system files are read only and packages are “layered” onto the system image through transactional upgrades. Most of the packages you want to install should be in containers like flatpak (for gui) and distrobox (for terminal). This keeps the base system clean and small and doesn’t get “bloated” like other mutable OS’s.