React vs Next.js: When to Use What?

React is the UI library. Next.js is a framework around React for routing, rendering, and production concerns.

Use a Vite/React SPA when the product is behind auth, has little SEO need, and is mostly interactive on one origin.

Use Next.js when you need SEO, mixed static/dynamic pages, or a server that can safely talk to APIs and mail without exposing secrets.