וכל פטר חמור תפדה בשה
שמות יג יג
To redeem every first born male donkey with a lamb
The Yisrael takes a lamb and gives to the Kohen in redemption of the donkey. The donkey belongs to Hashem as explained in Mitzva 18, Hashem gives it to the Kohen. Therefore the Yisrael must redeem it from the Kohen. If he doesn't have a lamb, he may redeem it with the value of a lamb. The lamb is not Kadosh in the hands of the Kohen, nor the donkey in the hands of the Yisrael.
That one has a first born.
Males and females. Yisraelim but not Kohanim, Leviim .
In Masechta Bechoros
So that we always remember the miracle Hashem did for us in Egypt, that he killed the first born of the Egyptians who are compared to donkeys.