To rebuild the client files, you need at least Node v14.
The source of the client files are located in ./app/client/src
npm run build will compile the source files there into ./app/client/public/. This directory is considered to be volitile and is deleted every time npm run build is invoked.
WebPack is used for building and the configuration is located in ./app/scripts
If one wishes to make changes to the javascript, the html, or the css it should be done in ./app/client/src and then complied using npm run build
For development purposes, you may also utilize npm run builddev which will not minimize the source and allow you to more easily troubleshoot while making customizations.