This is a great question! It’s a little complicated, but to simplify, they turn into various ligaments and other small blood vessels. The inside portion of the belly button is called the umbilical vein and the umbilical arteries. The umbilical vein turns into a ligament called the round ligament of the liver. The umbilical arteries turn into some arteries as well as the medial umbilical ligament. https://en.m.wikipedia.org/wiki/Umbilical_vein
See the table: https://www.med.umich.edu/lrc/coursepages/m1/embryology/embryo/18changesatbirth.htm
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