How would a watch measure blood pressure and blood oxygen?

It is a $40 ish dollar watch. Just flashes green and gives me numbers? I tried looking at the FaQ but it doesn’t make much sense. It also tracks heart rate.

The FaQ only talks about the heart rate. Here is the answer to “What does the green light do?”

“The green lights are used for monitoring your heart rate.The way this is done is through a process called photoplethysmography. The green light shines into the skin where your blood cells are passing by the area. There is an optical sensor that detects the passage of blood and then estimates a heart rate.”

Could this also be how they do the blood pressure + oxygen levels? I got this watch to track my sleep + steps, so this feature seems cool but also shady.

this is the watch