It does not need to be done in the US. To get an FAA unrestricted ATP certificate, you need to build on top of other FAA certs, but you can use experience gaine

It does not need to be done in the US. To get an FAA unrestricted ATP certificate, you need to build on top of other FAA certs, but you can use experience gained worldwide to satisfy all but a trivial amount of specifically called out instruction hours. If a pilot comes in with enough experience, it's a quick process to convert their foreign certs to FAA certs (which usually involves checkrides).