I mean "input files, a build step, output files" is the trivial part of this. I've made my own build systems which work the same way, it's not especially hard. But if you wanted to use my build system to make a Linux rootfs you'd find that to be a monumental effort since you'd have to do everything yourself.

It's everything built around the build script language that's relevant. How easy is it to make a Nix config which produces a rootfs with systemd, glibc, coreutils, an OpenSSL server, pam, a dbus broker, the necessary getty services, etc etc etc which makes it a Linux system? That's the question I'm asking.