Baby’s are made of materials floating around in their gestational parent’s blood. And guess what breastmilk is made of… It’s made of blood. Your body literally takes your blood and turns it into milk.
So until they start eating food, a breastfed baby is made entirely of their mother’s blood plus a single sperm.
Well, women are born with all the eggs they will develop over the course of their entire life, so in a sense a baby is made up of whatever their maternal grandma was eating during their pregnancy