You can transfer files between an Angular client and a Node.js backend in a single codebase by using server-side rendering (SSR) with Angular Universal. The SSR process involves sending the rendered view from the server to the browser, which then boots up the Angular application. The Angular application uses this approach to share data between the server and client. To manipulate files on the server, you can use the `fs` module in Node.js and pass a reference to the file list manipulation function to the Angular application using the `LIST_FILES` provider. This allows the Angular application to access the file system without running the `fs` module in the browser. The approach is secure because it only passes references to functions, not sensitive data like directory structure information.