Sforno on Numbers 8:22/ספורנו על במדבר ח׳:כ״ב

1כאשר צוה ה' את משה על הלוים, G’d had commanded that the task of the Levites would be to stand guard, to form the choir chanting the psalms and to carry the parts of the Tabernacle whenever the situation called for this. They would be assigned their immediate duties by Aaron and his sons.

2כן עשו להם, Aaron and his sons would organise them, allocate to them their specific duties.