Sounds like the only immutable part is the non-writable filesystem of the root partition, which is updated by having a live and non-live copy (A/B partitions) with the live updating the non-live and then switching on reboot. Similar to how Android works with its read-only partitions.

From a whole filesystem perspective I think it's not accurate to call this immutable though, as you can presumably work around this with bind mounts that can be used to mutate (but not persist) any part of the read-only filesystem while the system is still running.