This text explores the development and implementation of City Brain, a sophisticated cloud-based smart city infrastructure pioneered by Alibaba. Originally designed to alleviate chronic traffic congestion in Hangzhou through AI-driven signal control and video analysis, the system has expanded to manage public health, emergency services, and urban utility efficiency. The narrative details the historical struggle and ultimate success of lead engineer Wang Jian, who secured the unwavering support of Jack Ma to establish Alibaba Cloud as a global technological powerhouse. Today, this framework serves as the backbone for over 500 Chinese cities, integrating diverse IoT data to streamline resident services via centralized QR code systems. By transforming urban centers into interconnected digital ecosystems, City Brain represents a fundamental shift in how modern municipalities function and solve complex logistical challenges.