Jake Michaels / The New York Times
Nile Niami, a B-movie producer-turned-developer, in the hot yoga room in his house in Los Angeles, Nov. 27, 2017. While Gilded Age mansions were built as family legacies to be passed down to future generations or endowed to universities, tech-centric, ultramodern glass-and-marble behemoths like the homes built by Niami are designed for living in the moment.