Create a comprehensive step-by-step course designed for junior developers to learn React Native along with Expo. The course should focus on building reusable UI components and cover the following key topics: 

1. Setting up the development environment for React Native and Expo.
2. Designing and implementing a functional home page.
3. Creating authentication pages for user sign-up and login.
4. Developing a simple content page that fetches data from a server.
5. Building a chat page, incorporating an AI SDK or a suitable library for real-time messaging.

Ensure each module includes clear explanations, coding examples, and best practices for reusability and maintainability of components.