Rashi on TanakhJoshua 20

Until he stands, etc. for judgement. If he is acquited from exile, he is discharged. If he is sentanced to exile, he is returned to the city of his refuge, and he must reside there until the death of the Kohein-godol.

And on the other side of the Jordan, from Yericho eastward. [These cities were set aside] during the days of Moshe, as it is said, “Betzer in the wilderness.”¹Devorim 4:43.

That were set aside. They were prepared and set aside for this.