Six circles, each with a fixed position and a radius of 35, appear across the screen and react to mouse clicks. Clicking a circle changes its fill to white, adds a thick black outline, and brings it to the front, while all other circles revert to a plain black fill. Built with D3 v7, the example uses the `join` API to create and manage the SVG circles, with a unidirectional data flow (`udf.js`) that maintains the selected circle’s ID in state. The interaction is set up via a custom `setupClickInteraction` function that assigns a `click` event listener to each circle.