App.jsx and CSS Files

The root component. Every other component is placed inside it (directly or through routes).

import './App.css';

function App() {
  return (
    <div className="app">
      <h1>My App</h1>
    </div>
  );
}

export default App;

index.css holds global styles (fonts, body). App.css holds styles for App. Importing a CSS file in a component adds it to the page; the styles are still global, so use clear class names or CSS Modules (Button.module.css) to scope them.