stage-2

$ nix-build -A stage-2
/nix/store/rbfl5i46c62fy8yy1kk23i4h6xxg0gf8-stage-2

stage-2 is a script defined as config.system.build.bootStage2. It is the init script found in the rootfs, which is built as a squashfs image from toplevel. See below.