There are many works on this topic which are of academic nature? Oura (as in Oura Ring) is referring to some of those and is also holding patents on the subject

There are many works on this topic which are of academic nature? Oura (as in Oura Ring) is referring to some of those and is also holding patents on the subject. I wonder how Oura is dealing with this as their ring also has an SpO2 sensor.