TL;DR: Learn React’s component model and state flow before learning framework features. Then add Next.js routing, server and client boundaries, data fetching, forms, caching, testing, and deployment.
React First
Build a small interface with components, props, local state, controlled forms, effects, and custom hooks. Learn to keep state near the feature that owns it and to derive values instead of duplicating them.
Then Learn Next.js
Add file-based routing, layouts, loading and error states, server components, client components, metadata, and server-side data access. Understand why a component needs the client boundary before adding it.
Build One Real Project
Create a small issue tracker with authentication, search, pagination, optimistic updates, validation, and an empty state. Add tests for the important user flows and deploy it.
What to Measure
Check accessibility, page performance, bundle size, error states, and server response time. A production-ready React skill includes quality and debugging, not only JSX.