Only for the updates of the base OS. Updates to flatpak apps and distroboxes don’t need an update

And the actual download and install happens in the background before the reboot

So a 20 second reboot is hardly a huge cost for a much more solid update mechanisms

I'm not the person you're replying to, but what annoys me about this on Fedora is that I use full disk encryption in my notebook, so I need to type the password after the reboot for the updates and then one more time to get it running again. I wish it could deactivate all services and somewhat return to a state similar to early boot in order to apply the updates, rebooting only once afterward. This would be especially useful for shutdown updates, in which case no password would be necessary.

I was hoping that systemd's soft reboot function would help a lot here. It would make it unnecessary to request password when using full disk encryption and would require less downtime. Last I checked it didn't work in Silverblue though.

The same applies on Steam Deck. This is a thing with immutable systems.

if kernel is updated the OS would have to be rebooted anyway unless live patching is configured. Rebooting after an update is probably more common & less annoying than one would think.

Which update? The design of the system is so that there's minimal things installed that require a layer update.