A patent that Huawei filed last year has shed light on how the Huawei Watch D measures blood pressure. Seemingly, Huawei's approach relies on three sensors, with two measuring your wrist thickness and fat levels, which a smartwatch uses to correct the blood pressure data it receives.
https://www.notebookcheck.net/Huawei-patent-describes-how-blood-pressure-monitoring-could-work-in-the-Watch-D.578994.0.html