Cross-Platform Learning-Focused App Build

I’m ready to turn my wire-frames and feature list into a real, installable mobile app that runs on both iOS and Android from a single codebase. Because my main goal is personal learning, I want every part of the project to be structured so I can study and extend it later... (Budget: $15 - $25 USD, Jobs: Android, Documentation, Flutter, iOS Development, iPhone, Java, Mobile App Development, React Native)