React tells you how to describe a UI as a function of state. It says almost nothing about how a request reaches your server, how HTML gets generated before that UI ever runs in a browser, or how your code gets split into files a browser can actually download at a reasonable size. Next.js is the layer that answers those questions — and the friction most people hit isn't the API surface, it's the mental model shift: code that runs on a server by default, a caching layer with its own opinions, and a directive ("use client") that draws a boundary rather than flipping a switch on one file. Work through this in order, or jump to whatever section you need.