React Day 2 dives into the core elements that power React applications: JSX, components, and props. JSX, a syntax extension that allows developers to write HTML-like code within JavaScript, enhances code readability and maintainability by seamlessly blending markup and logic. Unlike HTML, JSX is transpiled by tools like Babel into React.createElement() calls, which create lightweight descriptions of UI elements.

This process is crucial for developers to understand as it affects how React components are rendered and managed. Functional components, introduced post-Hooks in React 16.8, offer a simpler, cleaner way to build UI by avoiding the complexities of class components. They are particularly powerful when paired with hooks, which provide state and effects management.

Props, short for properties, facilitate the flow of data from parent to child components, making them dynamic and reusable. By passing props as attributes in JSX, parents can efficiently control the behavior and appearance of their child components. Component reusability and composition are emphasized, promoting the 'Don't Repeat Yourself' (DRY) principle.

Developers are encouraged to design components that accept varied props, enabling them to be used in multiple contexts with minimal duplication. Composition, the practice of nesting components to build complex UIs from simpler ones, is achieved using the special children prop, allowing for flexible and modular code structures. These concepts are illustrated through real-world scenarios, such as dynamic post previews in a blog app or personalized greetings in a messaging system, demonstrating how React's principles can be applied to create efficient and maintainable applications.