Prepare the apples. Peel and core the apples, then cut them into slices measuring ½-inch to ¾-inch thick. Transfer the slices to a large bowl, pour the whole milk over them, and toss until coated. Set the bowl aside.
Make the flour coating. Add the all-purpose flour, cornstarch, 1 tablespoon of granulated sugar, and 1 teaspoon of cinnamon to a pie plate or another shallow dish. Mix until evenly combined.
Mix the cinnamon sugar. In a separate small bowl, stir together the remaining 3 tablespoons of granulated sugar and ½ teaspoon of cinnamon. Set this mixture aside for finishing the fried apples.
Coat the apple slices. Remove 6 to 8 slices from the milk at a time, allowing the extra milk to drip back into the bowl. Transfer the slices to the flour mixture and gently toss until every side is evenly covered. Set the coated apples on a large plate or tray. Continue with the remaining slices.
Prepare for frying. Pour ¼ cup vegetable oil into a medium-sized skillet and place it over medium heat. Set a rimmed baking sheet or large plate lined with paper towels beside the stovetop so the cooked apples have a place to drain.
Fry in batches. Add 6 to 8 coated apple slices to the skillet at a time. Fry for 1 to 1½ minutes per side, until the coating becomes lightly golden. Move the finished slices to the paper towel-lined baking sheet or plate.
Add the cinnamon sugar. While the fried apple slices are still warm, sprinkle them with the prepared cinnamon-sugar mixture. Transfer them to a serving plate.
Finish the remaining batches. Continue frying, draining, and coating the rest of the apple slices with cinnamon sugar until all of them are cooked.