How to use American Apple Watch in Japan

Set up and pair your Apple Watch with iPhone

To use your Apple Watch with watchOS 9, you need to pair your watch with an iPhone 8 or later with iOS 16 or later. Setup assistants on your iPhone and Apple Watch work together to help you pair and set up your watch.

Turn on, pair, and set up your Apple Watch

1. Put your Apple Watch on your wrist. Adjust the band or choose a band size so your Apple Watch fits closely but comfortably on your wrist.
2. For information about changing the band on your Apple Watch, see Remove, change, and fasten Apple Watch bands.
3. To turn on your Apple Watch, press and hold the side button until you see the Apple logo.
4. Bring your iPhone near your Apple Watch, wait for the Apple Watch pairing screen to appear on your iPhone, then tap Continue.
5. Or open the Apple Watch app on your iPhone, then tap Pair New Watch.
6. 
7. Tap Set Up for Myself.
8. When prompted, position your iPhone so that your Apple Watch appears in the viewfinder in the Apple Watch app. This pairs the two devices.
9. Tap Set Up Apple Watch, then follow the instructions on your iPhone and Apple Watch to finish setup.

To learn more about your Apple Watch while it’s syncing, tap Get to Know Your Watch. You can learn what’s new, view Apple Watch tips, and read this user guide, right on your iPhone. After your Apple Watch is set up, you can find this information by opening the Apple Watch app on your iPhone, then tapping Discover.

For more information Set up and pair your Apple Watch with iPhone - Apple Support

people report that the eSim of usa, canada and mexico does not work in the rest of the world,

the gps only watch should work but the warranty would likely only be valid in usa