React vs Flutter vs Angular
All 3 are open-source projects and can be found on GitHub
We can measure somehow popularity based on stars:
React > Flutter > Angular
React is from Facebook, Flutter and Angular are from Google.
The key difference between Facebook and Google is that Facebook has one single web stack (React + Hack), but Google has many different stacks. Flutter and Angular are just two out of many front-end frameworks.
React is for web, its sister project React Native is for mobile apps. React is only used for "view" (UI components), it needs to work with other projects (like Flux / Redux) to build a fully functional website.
Angular is also for web, unlike React, it is a "full" front-end framework.
Flutter was originally only for mobile apps, but later extended to web apps with the release of Flutter 2 in 2021.
Angular version 1.x was called AngularJS and has been deprecated; version 2+ were called Angular (no "JS") and had 2 flavors: Typescript and Dart. With the release of Flutter (also based on Dart), AngularDart is now in maintenance mode, leaving AngularTS as the only flavor.
React is still the most popular front-end framework, and Facebook will likely keep investing in it. However Google keeps killing its projects and products (the most recent example in the front-end world is PolymerProject), it's hard to predict the fate of AngularTS and Flutter, especially that Google's most mission critical sites are not using either. Flutter and Dart sound promissing since they are key parts of the new Fuchsia OS, but again Fuchsia's fate is quite unpredictable.