On my Samsung phone (6+ years old???) it came with a “health app”. One of the things it does is measure your stress level. Put your finger on the camera light monitor and press the button. It measures your pulse and your O2 level.
They say it is within about 2% of being accurate, so that’s good enough for me. Less than 90%(?) is call the doctor time.
My phone has the same thing. But some here say its not reliable enough.
I tried it last night. My index finger read 94 and 96. My middle finger (why not?) read 98. I quit while I was ahead.