And thou shalt give the Levites unto Aaron and to his sons: they are wholly given unto him out of the children of Israel.
Numbers 3:9 KJV. commentary https://bible.com/bible/1/num.3.9.KJV
So by this declaration Aaron and his sons where made automatic leaders and heads over the rest of levites https://bible.com/bible/1/num.3.9.KJV
because all the firstborn are mine; for on the day that I smote all the firstborn in the land of Egypt I hallowed unto me all the firstborn in Israel, both man and beast: mine shall they be: I am the LORD.
Numbers 3:13 KJV commentary https://bible.com/bible/1/num.3.13.KJV………
That makes sense, because in reality they are dead or were supposed to be dead.
But he has he kept alive for himself.
There is similar stuff in the NT that says we are crucified with christ nevertheless we live yet not I bust christ lives in me
Galatians 2:20 KJV
I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me.
So since all the first borns were his he chose by the law of substitution to replace them with Levi and for parents to pay a small token to redeem their first borns.
Also they were to present them to the lord as a reminder of the works of the lord https://bible.com/bible/1/num.3.13.KJV
NOTE 2
All the first borns where set aside, death had come upon them and they had survived because they were hallowed, God had set them apart.
They had been touched by the divine. They could no longer be common, the were hallowed.
That was how God saved them and preserved their lives from the angel 😇 of death ☠. By hallowing them https://bible.com/bible/1/num.3.13.KJV