ReactJS is a JavaScript library used for building user interfaces. Instagram leverages ReactJS to create its infinite feed and IGTV features. Here's how it works:

1. Virtual DOM: ReactJS uses a virtual DOM (Document Object Model) that allows it to update only the necessary parts of the UI without reloading the entire page, making the user experience faster and smoother.

2. Components: ReactJS breaks down the UI into reusable components that can be easily managed and updated. For example, each post in Instagram's infinite feed can be considered a component that can be easily updated as new posts are added or removed.

3. React Native: Instagram uses React Native to build its mobile app. React Native allows developers to build native mobile apps using ReactJS components and syntax, making it easier to maintain consistency across platforms.

4. State Management: ReactJS uses state management to keep track of the changing data in the app, such as the number of likes, comments, and views on each post. This data can then be easily passed down to the relevant components to update the UI.

5. Infinite Scroll: ReactJS can be used to implement an infinite scroll feature that automatically loads more content as the user scrolls down the page. This is what enables Instagram's infinite feed, allowing users to scroll through an unlimited number of posts.

6. Video Playback: ReactJS can also be used to implement video playback features, such as the video playback in IGTV. The video component can be easily updated with new videos and video data, such as views and likes, can be tracked using state management.

In conclusion, ReactJS provides a fast, efficient, and modular approach to building user interfaces, which makes it an ideal choice for Instagram's infinite feed and IGTV features.