the npm application is under src/main/frontend. This is a npm application, using React.
https://sourceforge.net/projects/nvm-for-windows.mirror/postdownload
Maven copy all files under target. Then, when the application starts, the browser access localhost:8080 (default SpringBoot port number, under src/main/resources/application.yaml) SpringBoot return the index.js present under target which is the React application.