Angular SSR file server.ts and file main.server.ts

I am using Angular SSR version 19. On the official Angular website https://angular.dev/api/ssr/node/CommonEngineRenderOptions they write that:

Note: In Angular v17 and later, server.ts is no longer used by ng serve. The dev server will use main.server.ts directly to perform server side rendering.

When I create a project, two files server.ts and main.server.ts appear in the project. The server.ts file contains the code:

import {
AngularNodeAppEngine,
createNodeRequestHandler,
isMainModule,
writeResponseToNodeResponse,
} from '@angular/ssr/node';
import express from 'express';
import { dirname, resolve } from 'nodeath';
import { fileURLToPath } from 'node:url';

const serverDistFolder = dirname(fileURLToPath(import.meta.url));
const browserDistFolder = resolve(serverDistFolder, '../browser');

const app = express();
const angularApp = new AngularNodeAppEngine();

/**
* Example Express Rest API endpoints can be defined here.
* Uncomment and define endpoints as necessary.
*
* Example:
* ```ts
* app.get('/api/**', (req, res) => {
* // Handle API request
* });
* ```
*/

/**
* Serve static files from /browser
*/
app.use(
express.static(browserDistFolder, {
maxAge: '1y',
index: false,
redirect: false,
}),
);

/**
* Handle all other requests by rendering the Angular application.
*/
app.use('/**', (req, res, next) => {

angularApp
.handle(req)
.then((response) =>
response ? writeResponseToNodeResponse(response, res) : next(),
)
.catch(next);

});

/**
* Start the server if this module is the main entry point.
* The server listens on the port defined by the `PORT` environment variable, or defaults to 4000.
*/
if (isMainModule(import.meta.url)) {
const port = process.env['PORT'] || 4000;
app.listen(port, () => {
console.log(`Node Express server listening on http://localhost:${port}`);
});
}

/**
* The request handler used by the Angular CLI (dev-server and during build).
*/
export const reqHandler = createNodeRequestHandler(app);


And how to use the main.server.ts file is not entirely clear because it is empty. I copied the entire code from the server.ts file and pasted it into the main.server.ts file and it does not work. I do not know why. Please tell me what I am doing wrong? I am trying to make the main.server.ts file work as it is said on the official website:

Note: In Angular v17 and later, server.ts is no longer used by ng serve. The dev server will use main.server.ts directly to perform server side rendering.

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