Can a website work with both simple JavaScript and React

Yes. This is exactly the problem that Facebook aimed to solve by creating React. They initially only used React for parts of the comment system and their chat.

You can create a React component and use it in only part of your website. You just need to provide it a place for it to mount. That is, a div where you are mounting your application.

Check the documentation for React DOM and the integration guide for more information about rendering to the DOM. The basic idea is to create your component then use the following code to render it:

ReactDOM.render(element, container[, callback])

From the React docs:

React can be used in any web application. It can be embedded in other applications and, with a little care, other applications can be embedded in React.

To integrate React with other libraries or frameworks, check out the React integration guide: https://reactjs.org/docs/integrating-with-other-libraries.html

Yes you can create global window renderer functions from react library and call it from vanilla js or some other third party library like jquery .You just need to include the minified js to call that function.

Some thing like below on react

const app = (parameter1,parameter2) => (
          <YourComponent parameter1={parameter1} parameter2={parameter2}/>
);





window.renderYourComponent = function (p1, p2) {
    render(app(p1,p2), document.getElementById('your_div'));
}

On your vanilla js

 <script type="text/javascript" src="your_minified_react.js"></script>
 <div id='your_div'></div> 
 window.renderYourComponent("p1","p2");

Everything will be rendered inside 'your_div' and you can work separately on react

Yes, You can make multiple React apps for a single website inside the Laravel framework. The Laravel often uses a blade template for frontend UI. What you need to do is build the React app and integrate the built React app with the Laravel blade. You can copy the whole built contents of the React app into the Laravel home blade, for example. In addition, inside React to import custom Javascript files, you can use several ways like script tags and also webpack copy module.