Pulse Ox is used to make medical decisions. You may not think that a reading that drifts from say 92-95% is that interesting, but that can influence the course of action a doctor takes can can definiately influence patent outcomes.
It depends. If (and it's a bif if) Apple are still infringing, then Massimo can be awarded damages even after the expiry of their patent to cover the damages incurred whilst it was active
It comes down to effort and likely outcomes. If Masimo sued one of the no-one Chinese brands, selling $10 units, the liklihood is the company would declare bankrupcy and Masimo would get no damages. Apple on the other hand, is a large entity that isn't going anywhere, selling infringing devices costing multing hundred dollars, so it is more likely that a successful lawsuit would lead to substantial damages paid to Masimo.
If you or a friend or relative have had a hospital stay, the odds are a Masimo product has been used.
Those cheap finger units aren't medically grade. There are strict certifications that medical devices have to go through, so even if the cheap units work in the same way and use the same components, if the variation/tolerances of the device haven't been certified they can't be used in medical settings.