unpkg is a fast, global CDN for everything on npm.
The project is maintained by React Training with sponsorship from Cloudflare and Heroku.
The website was built using create-react-app. This is the app you see when you run yarn start. However, none of the package links will work.
To start the backend, use yarn run server. This will start the backend so the website (which is really just a static HTML file) can serve as a proxy for package requests.