The term “giant” is a very poor translation of what is said in the mythology. The term used in Norse mythology is Jötunn (English Ettin), from proto-Nordic *etunaR, which means “hungry” or “big eater”.
Ýmir can be found in the womb of the mother, growing from the fertilized egg. He becomes the sky (amniotic bag), the placenta, the sea (amniotic liquid) and he constantly drips new Ettins (the amniotic fluid). Thus the entire world around the foetus is made from his body.
Then the gods cause a great flood (as the water goes) and cast him down into “the abyss”, as the child is born. The placenta is the twin that is “decapitated” when the child is born. He dies, but the child lives on.
He is called an Ettin because he is “hungry” and feeds off the mother. The placenta takes nourishment from the mother, and transfers it to the child. He is thus a monster, an ogre, to the mother, who has to control it, keep it in check, in order to survive the pregnancy.
Do you still don’t know enough, or what?
NB. This is a reproduction of an answer I wrote on Quora (Varg Vikernes) in July, 2019.