There's a tag system and data metrics matching system. Based on your comments, watch history, audience crossover between channels, watch time, and subscriptions, and data collected from your Google account (searches, apps opened in Android, etc.) it will generate a list of tags that are visible at the top of the home page. For example, some of my current ones are Electrical Engineering, History, Film Criticisms, and Scientific Explanations. The tags are constantly changing based on the factors mentioned above.