A smiley face made of bar-chart rectangles changes its expression based on population data from `data.csv`, which the code loads with D3's `csv` function. The visualization uses D3.js v5 to select the SVG element and bind the data to `rect` elements via `selectAll('rect').data(data)`, though the example only defines this rendering step without an explicit face-drawing function. The face itself is implied through the DOM manipulation pattern. The page includes a video explaining the process, and the code imports `select` and `csv` from `d3`.