ReactJS vs NodeJS - Why do I need to create both?

NodeJS is just a runtime that allows you to run javascript code outside of the browser.

In order to compile and transpile the react JS app, they use webpack and other tools which runs over NodeJS.

NodeJS will serve as your backend, whereas ReactJS will create the interface/UI where you can actually manipulate your server (nodeJS). So first you'll write your NodeJS server or API. You don't need to use ReactJS to create a frontend that would interact with your node server, like you said you can use NodeJS to create your views as well through a different library. ReactJS is just one choice of many for the front end of your NodeJS app.

NodeJS is not just regular javascript, it is a javascript runtime that sits on top of a C++ engine called V8, provided by Google. Node executes outside the browser, whereas React / Vue / Angular / etc are in-browser javascript frameworks.

React is a whole separate animal; it is a framework that renders its own DOM in the browser. It is a javascript engine that is configured to optimize DOM manipulation.

While the development pattern of frontend and backend appear similar, they are doing different things. React is handling component lifecycles, applying dynamic style rules, processing in-browser data, and making API calls. Node is handling requests from the browser, coordinating access to the server's file system, managing network I-O, performing cryptographic evaluation, etc. Because of these different responsibilities, Node makes use of different dependencies (read: node modules) than a frontend framework.

Ultimately, Node and React communicate through HTTP calls (or, less frequently, through a WebSocket or SOAP protocol).

It would behoove you to read about how node works under the hood.

React is front-end library. It provides great tooling for creatiing user interfaces. And it creates a single page application. Which means when you open a react app. It does not reload and its really fast.

While you could also use nodejs and something like handlebars to create a website. But that website would be rendered on server and then served to the user. But its alot more than that. There are a lot of things that you want to do on the server. Like authentication. You want that part to be secure. So you keep it on the server.

Now the answer to the final part of your question.

For a fully functional app. You would use react to create user interfaces. And nodejs for creating an API which your react app will call.