What is the process for creating heatmaps and how can I customize them?

Hey, I just learned about heatmaps and how they are used in data analysis, but my sir didn’t provide me any code to plot it on my own and investigate it further, and when I looked for its code online, I became absolutely confused. Is anyone able to assist me in creating a straightforward heatmap like this? And also can anyone give me a customized heatmap and jot down what did they customize and how did it make the analysis easier?