If want to buy a new iPhone in USA I can change the carrier in other countries

Make sure you get an unlocked phone.

Note that Buying an iPhone in one country to use in another country can have many disadvantages:

Make sure it will work in your country! --> 5G and LTE. Find the iPhone that’s right for your country or region. - https://www.apple.com/iphone/cellular/

and https://en.wikipedia.org/wiki/List_of_LTE_networks

Discounted phones sold in some US locations may be locked to a carrier. You have to complete the contract with the carrier before they will unlock the phone. In the end you are not saving money and may have to wait a long time. https://discussions.apple.com/thread/252444302?answerId=254609062022#254609062022

Warranty restriction! - http://www.apple.com/legal/warranty/products/ios-warranty-document-us.html - "IMPORTANT RESTRICTION FOR iPHONE, iPAD AND APPLE TV SERVICE. Apple may restrict warranty service for iPhone, iPad and Apple TV to the country where Apple or its Authorized Distributors originally sold the device." <-- The warranty does not outright deny warranty coverage in other countries but there are many limitations to the effect Apple almost certainly will refuse to do repairs under warranty or even repairs at all except in the country of purchase. The device would have to be sent to a friend in the country of purchase to handle any repair claims because Apple will not handle international shipping.

Devices may also have tax added to the cost (e.g., in the USA prices do not include taxes), and you may have to pay import duty when bringing an expensive device into your country.

Apple will not ship to other countries and will not knowingly deliver to freight forwarders.

I personally do not sell phones. This is a user to user support forum.

Apple sells both locked and unlocked phones. The locked ones are part of a package deal with carrier service that make them ready to use when you walk out of the store.