React and GitHubPages component issues?

Question

GitHub

There are quite a few articles like https://dev.to/yuribenjamin/how-to-deploy-react-ap... describing how to use react and gh-pages.
And everything is fine ... until you start using components.
for example

import React from 'react';
import { BrowserRouter, Route, Switch } from 'react-router-dom';
import Main from './components/Main/Main';
import './App.css';
import TestPage from "./components/TestPage/TestPage";
import MainGHPages from "./components/MainGHPages/MainGHPages";


function App() {
  return (
    <MainGHPages/>
  );
}

export default App;

Creates just an empty page on githubpages.
And without components, react is useless a little less, a little completely.
What am I doing wrong?