React provides a way to build pages and components, Next.js provides the other pieces that make up a full-stack framework like routing, asset mgmt, and most importantly, server-side rendering. Getting SSR to work seamlessly with Webpack, Babel, and any other frontend tooling is a pain in the ass, Next.js makes it dead simple.

React is more of a library instead of a framework and it's just the view layer. Next.js is a framework(minimalist) for React.

https://github.com/zeit/next.js/

And what would be the "maximalist"/rich framework for React? (with lots of pre-made RAD components I guess).

React is nostly the view layer. Next provides everything else.

> Next.js is a minimalistic framework for server-rendered React applications.

https://github.com/zeit/next.js/

So a specialized framework based on another framework

React by itself isn't really a framework.