This is the Twitter Open Source website at https://opensource.twitter.dev.
This site is built with [hugo], using a custom built-in theme. Follow the standard instructions on the hugo website to [install] and [run] hugo. Just make sure to use hugo-extended, which has added support for Sass/SCSS stylesheets.
There are also some python scripts that run periodically to [update some repo data].
[hugo]: https://gohugo.io/
[install]: https://gohugo.io/getting-started/installing/
[run]: https://gohugo.io/getting-started/usage/
[update some repo data]: ./.github/workflows/update-data.yml