A circle follows the mouse cursor across a full-screen SVG canvas. The circle's position is bound to the mouse coordinates, updated on every mouse move event. The example uses React with a `useState` hook to manage the cursor position, and the ReactDOM `render` method to mount the component. D3.js is loaded but not actively used in the rendering logic, which relies solely on native SVG elements and React event handling. The circle is drawn with a fixed radius of 30 pixels.