T3-Turbo provides an easy method to achieve type safety while using Next.js and Expo, but it lacks a feature for shared authentication between applications. This guide offers a solution by integrating Clerk, a user management system, into T3-Turbo to enable authentication across platforms. Users need a free Clerk account and must configure various files, including the Next.js package.json and the Expo application, to implement Clerk's authentication capabilities. The process involves setting up environment variables, wrapping applications in `<ClerkProvider>`, and using middleware to manage authentication states. The guide also includes steps for creating sign-in and sign-up components using OAuth, specifically with Discord, and explains how to update API packages to use Clerk instead of the previous Auth.js setup. It highlights the importance of protecting routes and ensuring secure storage of authentication tokens, ultimately enhancing both Next.js and Expo applications with scalable and reliable user authentication.