In early December, Spotify launched its annual personalized Wrapped playlist with its users’ most-streamed sounds of 2019. That has become a bit of a tradition and isn’t necessarily anything new, but for 2019, it also gave users a look back at how they used Spotify over the last decade. Because this was quite a large job, Spotify gave us a bit of a look under the covers of how it generated these lists for its ever-growing number of free and paid subscribers.