First, they can be held in a Customs and Border Patrol (CBP) facility for up to 72 hours. Then, they will likely be transferred to a detention center run by the Office of Refugee Resettlement (ORR), where they could potentially be held for weeks and even months. After an unaccompanied minor is taken into ORR custody, the child may be moved throughout the U.S. to a facility that has bed space. Children over the age of 12 are usually sent to group homes. Children under 12 are usually in group homes during the day and foster families at night.
If an unaccompanied minor is transferred to an ORR detention center, ORR will try to find someone in the U.S. to take custody of the child. The more distant the relation (relative vs. friend vs. stranger), the more time it will take to screen the adult before releasing the child into their custody.