Geographic Clustering with HDBSCAN

Your smartphone knows when you are at home or the office. At least, mine does, and can even tell me when to leave to get at one of my common destinations on time. We all accept that our smart devices collect information about our preferences and send them over to the cloud for processing. These come back as recommendations for shopping, food, mating, and when to leave the office and head home. What is the magic behind inferring a usual location?