Indoor human sensing, recognition, and detection are crucial technologies for building smart environments like smart homes, retail spaces, and museums. Traditional vision-based and wearable sensor-based solutions face challenges such as privacy concerns and obtrusiveness. In contrast, radio-frequency (RF)-based approaches, especially those utilizing WiFi, offer contactless, cost-efficient, and non-line-of-sight alternatives. This paper surveys recent advances in WiFi-based human sensing technologies, focusing on nine critical applications in smart environments: WiFi imaging, vital sign monitoring, human identification, gesture recognition, gait recognition, daily activity recognition, fall detection, human detection, and indoor positioning. The paper highlights the role of Channel State Information (CSI) in enabling these applications and discusses future research directions that can drive the development of more intelligent, responsive environments.