Apps that can measure your blood pressure don't actually work..

You know those apps that require you to place your finger on the camera lens in order to measure your blood pressure ? umm yeah, someone explain to me how does that even work? No, let me tell you, THEY DON'T. I've seen people using these apps and believing that they actually work, even arguing with me about it, just how do you think that if you place your finger on your phone camera it can measure your blood pressure ? It can't, your phone's camera doesn't have that capability, it's a camera you take pictures with it not measure blood pressure (facepalm). An app does not give your phone new features like that, you can't just download an app for something that your phone can't do, the poor phone can be as smart as he wants if you download an app that says it can make you pancakes is not gonna work, your phone will not start to walk and make you pancakes... But then again, there are ghost detectors out there, dude that's so cool, an app that can make you detect GHOSTS with your phone. Technology has come so far... Even when i searched this topic on google i didn't really find much about the authenticity of this apps and please feel free to correct me if I'm wrong and explain to me how does your camera measure your blood pressure.