Flutter is a cross-platform framework that enables building applications for mobile devices (iOS and Android), desktop (Mac and PC), and web browsers from the same codebase.
.NET is one of the most reliable and embraced open-source frameworks. It's a cross-platform technology so applications built in .NET can run on macOS, Linux, and Windows - the same code can be used for the entire backend.
React is a battle-tested frontend technology with over 119K libraries available. It's designed for building user interfaces, particularly for single-page applications where UI updates are frequent. It's great performance is one of the biggest advantages.