While there is a global trend towards making cities more sustainable, the concept and operations of sustainable cities may not be well-defined in the literature, as the number and diversity of studies on the topic make it challenging to integrate all perspectives. This study adopts a service system perspective to explore the concept and operational structure of sustainable cities. Text mining techniques and unsupervised machine learning algorithms were used to analyze 3,500 scientific articles on sustainable cities related to service systems. This analysis identified 24 research topics that represent key areas of inquiry, and classified them into three functional domains of control, infrastructure, and policy. The classification was used to develop a conceptual framework that interprets sustainable cities as large-scale service systems that are enabled by digital technology. The framework highlights the roles of various stakeholders, and provides insights into how urban operations can be designed to support sustainability. The findings offer a structured foundation for interdisciplinary understanding and future empirical research in sustainable urban development.