The actual React implementation used to look very similar to what we've built so far, but with React 16 this has changed dramatically with the introduction of Fibers. Fibers are a name that React gives to discrete units of work during the render process. And the React reconciliation algorithm was changed to be based on small units of work instead of one large, potentially long-running call to `render`. This means that React is now able to process just part of the render phase, pause to let the browser take care of other things, and resume again. This is the underlying change the enables the experimental Concurrent Mode as well as running most hooks without blocking the render.
But even with such a large change, the underlying algorithms for deciding how and when to render components is the same. And when not running in Concurrent Mode the effect is still the same as React does the render phase in one block still. So using a simplified interpretation that doesn't include all the complexities of breaking up the process in to chunks enables us to see more clearly how the process as a whole works. At this point bottlenecks are much more likely to occur from the underlying algorithms and not from the Fiber specific details.
