A d3.js heatmap displays a matrix of values using the Inferno color scale, with each cell representing a combination of a group and variable from a CSV dataset loaded from GitHub. The SVG rendering includes band scales for axes, rounded rectangles, and a tooltip that shows the exact value on hover. The code uses d3.scaleSequential with d3.interpolateInferno, d3.axisBottom, d3.axisLeft, and mouse event handlers for interactivity.