I don't know exactly how it works, but I know it's based off views, votes, and comments in a single day. Generally, when you update your story, it's more likely to show under the popular section. That's because (you hope, anyway) you'll get some votes, views, and comments on your newest chapter. However, I also noticed that the "premium" stories are always in popular. I don't know if that's because their stories are always getting attention, or if the Tapas algorithm just favors premium stories, so they'll always show there. Regardless, I don't think it matters how the popular section for novels works. These websites (Radish, Wattpad, YouTube, etc.) will always cater to their premium/popular content creators, so the most us normies can do it build up our own brand, despite what these websites do.