They are mine and will take the place of the first-born Israelite sons.
"Dedicate to me the first-born son of every family and the first-born males of your flocks and herds. These belong to me."
(SEE 3:11)
"The Levites will belong to me and will take the place of the first-born sons; their livestock will take the place of the Israelites' first-born livestock.
Assign the Levi tribe to Aaron the priest. They will be his assistants
They are assigned to help Aaron and his sons,