React.js is a JavaScript library for building user interfaces, and it is changing the world of app development by making it easier to create complex and dynamic web applications. React allows developers to build reusable UI components, which can be easily integrated into a larger project.

React.js is changing the app development process in several ways:

1. Component-based development: React uses a component-based approach to building user interfaces, which means developers can create reusable UI components that can be easily integrated into a larger project. This improves the maintainability and scalability of the code.

2. Virtual DOM: React uses a virtual DOM, which improves the performance of web apps by reducing the amount of direct manipulation of the actual DOM. This allows developers to build fast and responsive user interfaces.

3. Server-side rendering: React can be used to render components on the server-side, which improves the performance and SEO of web apps.

4. Large ecosystem: React’s popularity has led to a large ecosystem of supporting libraries and tools, making the development process more efficient.

5. Easy to learn and implement: React is easy to learn and implement even for beginners, which makes it more accessible to a wider range of developers.

6. Cross-platform compatibility: React Native, a framework built on top of React, allows developers to build mobile apps for iOS and Android using the same codebase. This makes the development process more efficient and cost-effective.

All these features make React a preferred choice for building complex and dynamic web and mobile apps.

Use cases:

There are many mobile apps that use React Native, a framework built on top of React.js, to build their iOS and Android apps. Some examples include:

1. Facebook
2. Instagram
3. Airbnb
4. UberEats
5. Discord
6. Skype
7. Tesla
8. Bloomberg

These are just a few examples of the many mobile apps that use React Native to build their iOS and Android apps. With the help of React Native, developers can use the same codebase to build apps for multiple platforms, making the development process more efficient and cost-effective.