Besides React and Angular, there is another framework that you should consider if you like simple stuff and that VueJs. VueJs is like a mirror image/ child of React because every concept in React is available in VueJs and is usually simpler to understand. These React and VueJs complement each other so learning the two will help you understand one or the other framework.
The Backend: This is where your data is stored, processed and given back to the frontend for it to be displayed to the user. The backend runs on a server and frontend runs on the client i.e. browser.
When you now know your NodeJs, React/Vue/Angular you can proudly call yourself a Fullstack Developer. The journey is long my best advice is to brace yourself and be persistent and keep your curiosity juicy.