Smartwatches split on the operating system. RTOS watches run a lightweight real-time OS on a low-power MCU, days to weeks of battery, a fixed feature set, no app store. Android-based watches (full Wear OS or a forked Android) are pocket computers on the wrist: app stores, Bluetooth calling, bright screens, and a battery measured in hours. The overwhelming majority of the hireable ODM layer is RTOS, where the volume and the margins are.
As on the heart-rate page, the SoC sets the tier. A Nordic nRF52, Realtek or Ambiq Apollo core defines a slim BLE health watch; a MediaTek or UNISOC cellular chip defines a 4G kids or elderly watch with its own SIM. On top sit the sensors this catalogue shares with the strap makers a few kilometres away, optical PPG for heart rate and SpO2, accelerometers, GPS, and increasingly single-lead ECG.
The houses cluster in Bao'an and Longhua, Shenzhen, and divide by purpose rather than geography: industrial and IoT wearables, family-safety GPS watches, mass-consumer fashion watches, and medical-grade health watches and rings. Each offers OEM, ODM and white label with in-house ID/MD, PCB, firmware, app and UI, carrying CE, FCC and RoHS.
A caution worth stating plainly: consumer smartwatches are wellness devices, not medical ones, validate every CE and FCC ID against the official database, not a vendor PDF. The apex brands (Apple, Huawei, Xiaomi, Garmin, and Amazfit / Zepp, whose maker Huami sits in Anhui, not the Delta) are not hireable; this maps the layer beneath them.