An iPhone bought in the USA will have all features enabled that are part of the country specific iOS version.
However, if you buy an iPhone abroad, these devices are supposed to be used in the country of purchase, due to the different built in hardware, that has to support local carriers, especially if it comes to LTE.
So make sure that your carrier supports this model and all its features.
This article has a list of all iPhone models and the supported carriers:
iPhone - Supported 5G and LTE networks - Apple
Also remember, that the warranty for an Apple Watch can be restricted by Apple to the country of purchase.
In case of a warranty claim, you will have to bring the device back to the country of purchase, Apple Support does not ship internationally.
Legal - iOS Warranty Document Us - Apple
The same applies to your Apple Watch
Apple Watch - Carriers - Apple
Legal - Apple Watch Warranty US - Apple