To get into the topic map, click on the map pin icon in the nav bar. If you are on a specific topic page already, the topic map will orient to show you where that topic is located.
Click inside a topic circle to zoom in and see its subtopics, or click anywhere outside the main topic circle to zoom back out a level. When you see a topic inside a rectangle (instead of a circle), that's the end of the line – it won't have any subtopics. You can click on those "final" topics to read explanations or add your own.