Where's the connection between index.html and index.js in a Create-React-App application?

 https://stackoverflow.com/questions/41738421/how-react-js-index-js-file-contacting-index-html-for-id-references

https://stackoverflow.com/questions/42438171/wheres-the-connection-between-index-html-and-index-js-in-a-create-react-app-app