All of web.dev is open source. Here's the directory [1] for the Learn CSS content. The site is all based on Eleventy [2]. I don't know if Rob (engineering lead [3]) and team did any funny stuff for the Learn CSS infrastructure in particular but you can ping him on Twitter for questions.

[1] https://github.com/GoogleChrome/web.dev/tree/main/src/site/c...

[2] https://www.11ty.dev/

[3] https://twitter.com/rob_dodson

Thank you very much!