Saudi Arabia's mega-project portfolio is reshaping the operational logic of the built environment. Smart districts, high-rise mixed-use assets, hospitals, transport hubs, hotels and mission-critical public facilities are expected to perform as continuously optimised systems rather than static construction outputs. This review examines how digital twin applications can improve smart building operations, maintenance and risk management in Saudi mega projects under Vision 2030. The paper synthesises peer-reviewed literature from 2020 to 2025 and develops a building lifecycle framework linking BIM, IoT, building management systems, artificial intelligence, facility management platforms and governance controls. The central argument is that a digital twin becomes valuable only when it converts live building data into verified operational decisions: fault detection, energy optimisation, predictive maintenance, safety alerts, resilience planning, asset lifecycle analysis and executive risk visibility. The review identifies four implementation layers: data foundation, analytical intelligence, operational intervention and governance assurance. Findings show that digital twins can reduce fragmentation between design, construction and facility management; however, success depends on data quality, interoperability, cybersecurity, workforce capability, procurement discipline and ethical use of automation. For Saudi mega projects, the strongest value is expected in MEP-intensive assets where energy, cooling, lifts, water systems, fire safety, indoor environmental quality and equipment availability directly affect public value. The paper proposes a practical roadmap for staged adoption, beginning with priority assets and moving toward portfolio-level digital twin governance. It concludes that digital twins should be treated as socio-technical operating systems for smart buildings, not merely 3D visual models.