Is the oxygen sensor built into every Apple Watch? A new update today will enable it on "some" Apple Watches.

I was VERY excited today to get a message from Apple about the oxygen sensor functionality returning to the Apple Watch. Until I saw that the watch is collecting the data, but that it's transferring it to the phone and it can be viewed in the Health app. Not on the watch.

I live at 9,000 feet, and I depend on my Apple Watch to monitor my blood oxygen levels. We had also gotten my elderly father an Apple Watch to monitor his oxygen levels because of ongoing health issues. Visitors to where we live—in the mountains of Colorado near the ski resorts—also depend on this feature.

My question is this...do all Apple Watches have this functionality built in, and we're just waiting for an eventual update that can enable the hardware? I would actually be willing to pay a license fee to turn this feature on, and have held off buying another Apple Watch because of this missing functionality. My current watch is about four years old.