Build Basic Cross-Platform Mobile App

I’m experimenting with the full development workflow on Freelancer, so I’d like a modest proof-of-concept mobile app that I can study and extend later. The app must compile for both iOS and Android and include three working features from the start: secure user authentication, standard push notifications, and a placeholder in-app purchase flow.

Here’s what I need at the end:
• A clean, well-commented codebase (React Native, Flutter, or another cross-platform framework you prefer) that builds on current Xcode and Android Studio releases.
• A short README explaining environment setup, build steps, and where each of the three features lives in the code.
• Demonstration videos or screenshots that verify login, a test push notification, and a mock purchase going through sandbox/test mode.

Keep the interface minimal—think sign-in screen, a simple home screen, and one button that triggers the purchase flow—so I can focus on understanding the underlying logic. Feel free to suggest libraries or services (Firebase Auth, Expo push, RevenueCat, etc.) if they speed things up; I’m open to alternatives as long as they remain free or have generous trial tiers.

This is mainly a learning exercise for me, but I still expect production-quality structure and clear comments so I can follow along.