ifixit has taken the Apple Watch Ultra apart. The higher build of the model is primarily due to the significantly larger battery. It has a capacity of 542 mAH, which is almost eighty percent more than the conventional Apple Watch. Energy is the be-all and end-all for most functions.
The disassembly is not trivial according to ifixit and also caused damage in the video. In addition, ifixit assumes that it will be very difficult to make the watch waterproof again. It is therefore better to have the battery replaced by Apple, since the complexity seems to have increased significantly compared to a normal Apple Watch.
