Theirs isn't the only way and apple could have purchased the license to use it but opted to rip off mosimo instead. Notice that samsung and many other watch manufacturers with pulse ox aren't affected by this. Only Apple.

I've checked my samsung watch against actual medical pulse ox monitors I have access to and they're within 1% of each other in heart rate (within a couple bpm, at least) and oxygen saturation do they seem as accurate.

Also, Apple does not claim to be a medical device. Neither does samsung. They aren't opening themselves up to all the liability and extra hoops for that, so no one should be medically relying on them, even if they do appear to be working as such.