This happens for a variety of reasons, such as comparative advantage or economies of scale. Economic integration also facilitates knowledge transfer, bringing innovation to places that would otherwise not have benefited from these technologies. At the same time, just because trade leads to gains overall does not mean there won’t be some left worse off.
This has been studied since the early days of international economics. Recent surveys have identified groups that have suffered from international trading arrangements, at least relative to their national peers.3 These effects are not proof that international trade theories have failed – indeed they are part of the evidence that the theories are right.