From what you have said, I think my mental model for how life forms at the mammal-size level is wrong. The piece I was missing is that even for large organisms

From what you have said, I think my mental model for how life forms at the mammal-size level is wrong. The piece I was missing is that even for large organisms, new life is created by cells (eggs) already containing mitochondria dividing. New cells aren't created from scratch.