Sabrina's Doppelganger corners Theo, stating that she can turn him into a real boy. The idea that, because Theo transitioned into a boy does not make him a real boy. Her transphobia upsets Theo as he states, he doesn't need to be born a boy to be a boy.
When turtles are born, they absorb the minerals around them and the chemicals from that exact area. This enables them to lay their eggs in the same spot.