Apple Watch backups are made when the watch is unpaired from the phone.
You’ll experience this, for example, if you upgrade your phone but not your watch. You’ll un-pair your watch from your existing phone, before your upgrade, then re-pair it to your new phone. When you re-pair it, you will be asked if you want to restore from backup.
In any case, that’s the simple case, because it’s the same actual watch. It’ll work just fine.
I had a situation last week that was confusing & frustrating. It took two days, but I figured it out and the answer saved me hours.