If an intending immigrant is not currently in the United States, he or she cannot file for a green card in the United States via adjustment of status. Rather, he or she must generally process through his or her local consulate.
As a practical matter, the steps that would be taken in the United States as part of an adjustment of status petition (e.g., petition review, interview, etc.) are instead conducted abroad by a consular officer. If approved, the immigrant will receive an immigrant visa in his or her passport, which will allow him or her to travel to the United States. Shortly after entry, the immigrant will be mailed a green card.