React is a JavaScript library for building the user interface. It is maintained by Facebook and a community of individual developers and companies. React was created by Jordan Walke, a software engineer at Facebook.Facebook And Instagram Both Use React/React Native.

Single-Page Application: SPA is Type Of Application That Doesn't Have Pages.Yes! If You Have Clicked On About Button In SPA Your Page Have Not Changed Pages Yet Only Your View Of The Application Has Changed Just Like JS Show And Hide Property We Use To Show Things On A Click Of A Button.

Multi-Page Application: MPA is a Same Old kind of web application. This means that every time You Click On About Button it has to request a new page from the server and then render it in the web browser

React.js is considered one of the best JavaScript libraries for building user interfaces because of the following reasons:

1.Components-based architecture: React allows developers to break down complex UI into small, reusable components. This makes it easier to manage and maintain code.

2.Virtual DOM: React uses a virtual DOM, which is a lightweight in-memory representation of the actual DOM. This allows React to efficiently update the UI and results in faster rendering.

3.Performance: React is fast and efficient due to its ability to update the UI without reloading the entire page. This results in a better user experience.

4.Large community: React has a large, supportive community of developers who contribute to the library, build tools, and provide support.

5.SEO-friendly: React can help improve search engine optimization as it can render on the server-side, which is important for search engine crawlers.

6.Easy to learn: React has a simple and intuitive API that makes it easy for developers to get started and learn the library quickly.

Of course, the best technology for any particular project will depend on the specific requirements and use case. However, React's popularity and wide usage by companies such as Facebook, Airbnb, and Netflix are a testament to its robustness and usefulness as a UI library.