I relay the words of my housemate that's a nurse. How are they able to do this on simply your wrist when they use large machines with multiple sensors in hospit

I relay the words of my housemate that's a nurse. How are they able to do this on simply your wrist when they use large machines with multiple sensors in hospitals? My argument is that those machines are highly accurate, or at least the most accurate in order to diagnose. This extends to the other measurements too like respiratory rate and bloody oxygen saturation.