How do immutable distros work?

I'm really interested in trying out an immutable distro like Fedora Silverblue. And I get the concept that the base system is read only and you install software through flatpaks.
But how does this work in practice?

If I change a config file in /etc, will the changes survive a reboot?
How can I see or decide which parts of the system are immutable and which are not?
Can I install packages that aren't available as flatpaks, like more obscure stuff or command line tools?
What about ppd files for printing, firmware, drivers, fonts, icon packs, etc.? Where and how do those get installed?
Also, is it possible to replace or remove packages that are within the immutable part of the system?