Specifically, it is a fork of the react-scripts package. It adds Backpack component support and server-side rendering functionality (amongst other things).
In order to benefit from the ongoing open source development of create-react-app, this fork is kept up-to-date with the upstream repo. To aid this, we have preserved the existing repo structure, including all of the source code for the other packages that live alongside react-scripts. We do not modify any source code outside of react-scripts.
Files of interest: