Here Webpack is used to build react jsx.

webpack.config.js

• entryPoint: points to React index.js that resides in client/index.js
• outputDir : /static/bundle.js( after the build is complete the bundle.js go in this folder

app.js

app.use("/api/v1/users", userRouter)

• all the /api/v1/users request goes to userRouter

app.use("/*", indexRouter)

• all the / request goes to index.js router which in return route it to index.ejs
• in index.ejs react js bundle file is included , hence routing for /* will be handled by react routers
• in index.ejs the react component will be rendered inside <div id="root"></div>

similar to create-react-app we have public/index.html there , here we have index.ejs