The nix file is besides the point - it gives you a totally hermetic build environment. Not OP, but it’s the only way I know how to get gcc to use a static glibc. All you should pay attention to is that it’s using a static glibc.
$out is a magic variable in nix that means the output of the derivation - the directory that nix moves to its final destination
$out is a magic variable in nix that means the output of the derivation - the directory that nix moves to its final destination