iPhone 12 back glass broke within a week

There is no such thing as worldwide warranty and the back glass on iPhone is not covered under warranty as if it is broken, it's accidental damage and not covered anyway.

More bad news. The back glass is fused to the phone and cannot be replaced. The entire phone will need to be replaced. If you have a valid AppleCare+ plan you purchased with the phone, the cost is reduced. If you don't have a valid AppleCare+ plan, the cost will be about 60% of the price of the phone when you bought it.

Do you live in the country where the phone was purchased? If you don't, you'll have to return the phone in person to a store in the country where you purchased the phone.