Eg, how do I alter a file, say /etc/fstsb, in Fedora Silverblue, Nitrux, BlendOS etc?
I’ll answer for Fedora Silverblue as it’s the only one I’m confident about. So, by default, both /etc and /var are writable. Furthermore a lot of traditionally writable parts (like /home) are contained within /var as well. So say you’d want to edit /etc/fstab (which I’ve done in the past), then you’d literally do it the very same way you’d do it on non-‘immutable’ distros. So; copy (the content of) /etc/fstab, change whatever you want and sudo cp the modified file to /etc/fstab and you’re done.
Perhaps interesting to point out is that, on Fedora Silverblue, all changes compared to the pristine copy of /etc (which is kept in /usr/etc) are being tracked and can even be accessed with ostree admin config-diff. Note that ‘traditionally’ the contents of /usr has been one of the harder parts to modify on Fedora Silverblue and I’d argue the average Joe should not engage with it as it’s very easy to mess up. However, uBlue actually enables one to engage relatively easily with those harder to modify parts. And the amount of configurability it allows should definitely put anyone to shame that continues to posit that “immutable is inflexible”.