Like any web application, what is served by the server is an index.html . Then there will be few javascript files downloaded as required by the script tags in the index.html. files downloaded at start-up index.html Out of these files, main.js is important and it is the compiled version of the main.ts within the src folder. So main.js is the entry point and then it will call the following files in the below-depicted order. file call hierarchy So if we need to do any pre-Angular stuff to do some hacking, main.ts looks like the correct place. After main.ts app.module.ts will be invoked, and then the root component mentioned in the bootstrap section of the @NgModule decorator will be loaded. Refer https://angular.io/guide/bootstrapping#the-bootstrap-array Normally there will be a single component tree with a single root component and all other components nested within the root component. Once the br...