Apple announced the Watch Series 12 and the Watch Ultra 4 at its Surprise and Shine event, and almost all of the work went into the part of the watch nobody sees. The sensing system on the back has been rebuilt around a larger electrode surface, photodiodes arranged in a ring and bigger green LEDs. The point of all of it is frequency. The watch now takes a heart rate reading every five seconds, sixty times more often than before, and it samples heart rate variability every five minutes. Apple calls it the most accurate heart rate sensor in a wearable, which is Apple's measure rather than anyone else's. Both watches also gain Readiness, a score built from activity, vitals and sleep data.
The one change you can actually see is ceramic. The last ceramic Apple Watch was the Series 5, seven years ago, and it came only in white, per AppleInsider. This time there are two colors, and nothing else about the case moved. We wrote on September 7 that a new Readiness app was the real Series 12 news, on the strength of a string in the watchOS 27 code and nothing Apple had said. It shipped today, on the hardware it needed.