The documentation is very brief with this topic:
ng eject
ejects your app and output the proper webpack configuration and scripts
ng eject basically get-rid of angular cli scripts and introduce webpack scripts in package.json, underlying webpack.config.js file mainly for comprehensive way to manage the project and it will be completely our responsibility to manage the project configurations after this.
A comparison of package.json scripts before and after is below,
check https://medium.jonasbandi.net/to-use-angular-cli-or-not-187f87d0b550 for further read.