I would think one level down from React is the DOM API, and one level down from that is the rendering engine (i.e. knowing about how DOM operations affect repaints, whether repaints block the main thread, etc.

Language-wise, I'd say babel "javascript" is the highest level of abstraction. One level below is understanding what things like JSX and class properties transpile to. One level below is understanding JIT strategies (e.g. when hidden classes deopt), memory usage of various patterns, etc. One level below is understanding specific semantics of high level APIs (e.g. exact accuracy of a high res timer implementation), or regexp memory footprint by looking at browser source code.