Smart devices are utilized by billions worldwide. The ubiquity of consumer-grade smart devices provides opportunities to robustly capture real-world cognition. Intuition (NCT05058950) was a virtual, observational study that enrolled 23,004 U.S. adults in 18-months collecting longitudinal multimodal data via iPhone and Apple Watch using a custom research application that captured routine device use, self-reported health information, and cognitive assessments. The study objectives were to classify mild cognitive impairment, characterize cognitive trajectories, and develop tools to detect and track cognitive health at scale. The study addresses sources of bias in current cognitive health research, including limited representativeness (e.g., racial/ethnic, geographic) and accuracy of cognitive measurement tools. We provide study rationale and design, then describe baseline demographics, clinical, and neuropsychological characteristics stratified by vulnerability to cognitive decline. Initial results support the feasibility to detect mild cognitive impairment and describe at-risk cognitive health trajectories in an ecologically valid and demographically diverse aging population.