Sforno on Numbers 3:13/ספורנו על במדבר ג׳:י״ג

1כי לי כל בכור, as the Temple service used to be the duty of the respective firstborn in each family.ביום הכותי כל בכור בארץ מצרים הקדשתי לי כל בכור בישראל, although at the time when the firstborn in Egypt were killed, the Jewish firstborn had been guilty of death also, as had been the firstborn of that entire generation. They were singled out for this penalty as they were the most honoured people of their time, and as such served as role models for others. There was no special merit due to which they would have been spared, as in Genesis 19,15 where the angels warned Lot not to remain in Sodom lest he too would be killed on account of the company he kept. G’d, in sanctifying them, i.e. placing them on a spiritually elevated pedestal so that they would not partake in all manner of secular pursuits, justified sparing them at that time. Now that, due to the sin of the golden calf, they had forfeited this spiritually elevated status, they had to be “redeemed” in order to resume life on a more mundane level than that for which they had been destined. This is the meaning of וכל בכור אדם תפדה, “you shall redeem every human male firstborn.” (Exodus 13,13).We must not make the mistake of thinking that seeing that this verse was written before the sin of the golden calf, that the firstborn had already been subject to redemption at that time, i.e that G’d had despised them as not fit to offer the sacrificial offerings. On the contrary, the “redemption” of which Exodus 13,13 speaks, i.e. [without a simultaneous substitution of others in their place, Ed.] was only in order to permit these firstborn to also pursue a regular lifestyle when they would not be performing service in the Temple, etc. Without “redemption” every object that has been sanctified is out of bounds for any mundane use whether passive or active, as it would constitute a demeaning of its holiness. (compare Chulin 137)

2מאדם עד בהמה לי יהיו, from now on, humans have to be redeemed, animals to be slaughtered and their blood and fat offered on the altar.