Not unlike other methods of panfrying, stir-frying—usually—has one important difference: you cut the food into bite-sized pieces before cooking it instead of afterward. This makes for lightning-fast cooking.
Generally speaking, with home equipment it’s best to stir-fry most or even all of the ingredients separately, combining them only at the last minute. That’s the only way to brown each of them and to cook them by direct heat instead of simmering the pieces in their own juices, which is what will happen if you crowd them, no matter how high the heat. Most of these recipes reflect that reality.