React is a JavaScript library for building UI components. Next.js is a framework built on top of React that adds routing, server-side rendering, static generation, and other application-level concerns.
For a marketing website or SEO-sensitive application, Next.js is usually the better starting point since server-side rendering and static generation are built in, rather than requiring separate tooling.
Plain React (typically paired with a bundler like Vite) can be a reasonable choice for applications behind a login where SEO isn't a factor and you want more control over the build setup.
In short: it's less "Next.js vs React" and more "do you need a framework's opinions and built-in rendering strategies, or are you fine assembling those yourself."