The Apple Watch Series 3 has some significant hardware. It has a GPS. It also has gyroscopes and accelerometers.
More interesting is the plethysmograph.
This sensor hardware is what detects pulse but it is cable of much more. On its own it can also measure oxygen level. Combine with the accelerometers it can be used to compute blood pressure. Collectively, the Watch also has the necessary sensor data to measure EKG.
Those advanced medical measurements are possible but Apple hasn’t shown any inclination to implement the software.
One reason may be the FDA and the lawyers. Too bad. The Apple Watch could be the hottest accessory for our aging society.