Hey Pawan Maurya! I don't know much about react.js but I did find this on SoloLearn: https://www.sololearn.com/learn/11962/?ref=app
There are other posts as well on react.js. Just go to the courses and search in the search box.
I have no idea what flutter is though unfortunately.
• Creating mobile app with React Native — https://ideamotive.co/react-native-development-guide/
Swift/Objective-C for iOS and Java/Kotlin for Android? Why not writing only one version in React Native?
Burey 🍻 shares great stories 👌 if you want to stay up to date,😉
Top Picks: React Status
• Learn React — https://www.freecodecamp.org/news/want-to-become-a-react-developer-947c9a6dbb76/
• Full Course for Beginners — https://www.freecodecamp.org/news/learn-react-course/
• A React.js tutorial for beginners — https://www.freecodecamp.org/news/learn-react-js-in-5-minutes-526472d292f4/