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.
Tis the difference betwixt private and personal property, we don’t want your toothbrush but your toothbrush factory however…