actually they hackathoned this in PHP so probably
i think LEPP + Laravel + Inertia?
commit f36bdfc
https://dev-null.sh/about you can watch the full 2 day livestream on the development on the site.
/dev/nullAbout - /dev/nullHow /dev/null was built in the open: from the first commit on stream to the developers who joined and the people who shipped it.