main.js

import { createRoot } from 'react-dom/client';

// import component
import Second from '../components/second.jsx';

const bootstrapReact = () => {
   const root = createRoot(document.getElementById('insertReactHere'));  // parent for created element = "insertion point"

   root.render(<Second />);             // use defined imported component
}

window.addEventListener('DOMContentLoaded', bootstrapReact);