So (some) dinosaurs didn't have wings, but they did have arms. Basically (I'm putting this incredibly simply) the arms evolved into wings. The bone structures actually stayed pretty much the same anatomically, just changed shape and density. And as feathers evolved and the smaller and smaller species survived and the larger ones became extinct, over hundreds of millions of years and billions of generations the birds of today came into existence through natural selection. So basically they didn't grow wings, their arms changed function to become what we call wings. Vertebrates in general share very similar bone structures, and even in animals missing limbs, their skeletons still show evidence of what was once there. For example humans have tail bones, and whales and snakes have remnant bones from where their legs were on their ancestors. I hope that's helpful. I kept it as short and sweet as I could.