Both React Native and Flutter are pretty well-known in the cross-platform universe, and they are both great options for building mobile apps. Some context:
It’s a library created by Facebook to create hybrid-native apps (Android, iOS) using a single codebase which was written on top of React (https://react.org). This is a great option if you already have a background in React Web. This library compiles to Android and iOS but doesn’t offer support for web or progressive web apps.
It’s a framework by Google that allows you to create cross-platform mobile apps and desktop apps with a single codebase. Learning Flutter requires having knowledge of Dart (its programming language). This library is optimized for mobile apps. It also allows you to create web and desktop apps, but this capability is still in beta.
In this article some colleagues created an in-depth comparison of Flutter and React, their limitations and advantages. This is it: Flutter vs React Native: Everything You Need to Know - UruIT Blog.