Flat white is 2 espresso shots and equal parts steamed milk. A Latte is more steamed milk (idk the exact ratio if there is one), most places just fill up whatever latte cup they have which should be bigger than the flat white cup, and has a small amount of head/foam.
Biology definitions are not based in causation, it wouldn’t lay eggs just cuz we call it a bird. It might be like the anti-platypus tho, the only bird to give birth to live young