Building an API, beside a (React/Firebase) web app - reactjs

Question

I have a web app using React and Firebase Realtime Database.
The index.js file looks like the following:
import React from 'react';
import { userInfo } from 'os'
import ReactDOM from 'react-dom/client';
import { BrowserRouter as Router, Routes, Route } from 'react-router-dom';
.....
class RouterBlock extends React.Component {
constructor(props) {
super(props);
this.state = {
logdInSatus: false
};
}
.....
render() {
.....
return (
<Router>
<Routes>
<Route exact path="/" element={<TopMenu/>} />
.....
<Route exact path="/p1" element={<Component_1/>} />
<Route exact path="/p2" element={<Component_2/>} />
<Route exact path="/p3" element={<Component_3/>} />
.....
</Routes>
</Router>
);
}
}
It all works pretty much the way I wish.
From this point, here is what I want:
An API for a customer, which is going to allow him to get data from my DB for his own use.
My app is now accessible through a URL like:
https://example.com/myWebApp
The API for the customer(s) should be used via this:
https://example.com/myAPI
and it would return JSON data from my DB that the customer can then use.
The top file for the API would probably look something like (but I am not yet sure):
const http = require("http")
import { getDatabase, ref, onValue} from "firebase/database";
import { url } from 'inspector'
import { workerData } from 'worker_threads'
const db = getDatabase();
.....
const requestListener = function (req, res) {
res.setHeader("Content-Type", "application/json")
res.writeHead(200)
res.end(`{"message": "This is a JSON response"}`)
// Here data needs to be taken from the actual DB.
}
const server = http.createServer(requestListener)
server.listen(port, host, () => {
console.log(`Server is running on http://${host}:${port}`)
})
Finally, my question is about the routing for the API url. Should it be one of the routes found in index.js or something totally separated? If this is the case (a route inside index.js), what is the proper syntax that I should use for this route? What I tried did not work.
Since all my current routes use a React component, I suppose this one needs to be different, but I don't know how to handle it. Any relevant hint will be welcome.