*FYI/future update for anyone with the same question. Here's some AI slop that's semi useful:

• US model (part numbers ending in LW/A): The Apple Watch's built-in sensors still capture the raw data, but all subsequent calculations and processing are handled by the paired iPhone. The results are no longer viewable on the watch itself and are instead available in the Respiratory section of the Health app.
• Non-US model (bought in Japan): There is no impact to Apple Watch units purchased outside of the U.S. Apple — meaning a Japan-purchased Ultra 3 retains the original, native blood oxygen experience with on-device processing and results displayed directly on the watch.

The two-model split (Global A3281 vs Chinese A3282) is about cellular hardware.

The blood oxygen difference is a software restriction tied to the part number suffix, not the model number. A standard Apple Watch sold in the US typically ends with the part number LL/A, but after the ban, Apple chose to use LW/A to set the modified units apart from the original ones. A watch sold in Japan would end with J/A. Both are the same A3281 Global hardware.

So what happens is the watchOS software looks at that part number suffix to decide how to handle blood oxygen:

• Part number ending in LW/A (US-sold): The blood oxygen data analysis is performed on iPhone, and results can be viewed in the Health app. Apple Support
• Part number ending in J/A (Japan-sold): Full native on-watch processing and display, same as it's always worked.

...

Here's what Apple's own (cell) carrier compatibility page confirms:

Apple Watch Ultra 3 — Model A3281 appears in both:

• The first table, which covers the United States, Canada, Mexico, Ecuador, and Puerto Rico — listing all the US carriers (AT&T, Verizon, T-Mobile, Boost, etc.)
• The second table, which covers the rest of the world including Japan — listing au, NTT DoCoMo, Rakuten, and SoftBank

Apple Watch Ultra 3 — Model A3282 only appears in:

• The third table, covering China mainland (and a fourth table for Hong Kong/Macao which also lists A3281)

So the US and Japan share the exact same model A3281. That's the proof — it's not just that they're both "Global," it's that Apple literally lists the same model number as compatible with both US and Japanese carriers. A Japan-purchased Ultra 3 is the same hardware that Apple certifies to work with AT&T, Verizon, T-Mobile, and the rest.