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React Docs would be a good place to start:
https://react.dev/learn
I found the vercel react basics doc pretty good too:
https://nextjs.org/learn/react-foundations
Quick Start – React
The library for web and native user interfaces
React Foundations | Next.js
Learn the fundamental JavaScript and React concepts that'll help you get started with Next.js.
I personally did this one to get to know fullstack and the front-end is based on React https://fullstackopen.com/en/
Full stack open
Open online course on JavaScript based modern web development by University of Helsinki and Houston Inc..