Currently all smartwatch models sensors are integrated into the bottom of the device so that in this way the measurements are more precise, since as a general rule, unless we wear the device very loose, it is always in contact with our skin to take measurements. However, although it may seem the most logical option, it sparked a great debate in the Cupertino facilities when the first steps were being taken to create the Apple Watch, since many of the engineers were in favor of integrating the sensors into the straps of the device and not at its bottom.
But placing the sensors on the strap also has its share of logic, although it was not practical. According to Bob Messerschmidt, a former Apple employee who participated in the development of the Apple Watch, the idea of placing the sensors on the strap is due to the measurement issue, since if they were located in the lower part of the wrist ( right at the strap closure), these would be more correct than on top of it.
But this clashed with Apple's plans to create a wide range of interchangeable straps, belts that if they also had to integrate the sensors would skyrocket in price, if there are already some models that are very expensive for what they really are. The Apple Watch, like the iPhone, iPad and other products are devices that the company launches to do business and the sale of straps is bringing great benefits, although many are the users who prefer to make use of third-party straps manufactured in China that are sold in the market for a tenth of what the originals cost.