Preheat your oven to 350°F (≈ 175°C). Grease a loaf pan (commonly 8×4 or 9×5 inch) and line with parchment if you want easier removal. 
Toss the chopped apples with brown sugar and cinnamon. Some recipes let this sit a bit so the apples release some moisture and get coated well. 
In a bowl, whisk together flour, baking powder, and salt. 
In another bowl, cream butter and granulated sugar until light, then beat in eggs, vanilla, and milk. 
Fold the dry mixture into the wet until just combined. Be careful not to overmix. 
Pour half the batter into the loaf pan.
Add a layer of apples (half of the apple mixture).
Then either swirl in a cinnamon-sugar swirl or sprinkle the brown sugar cinnamon topping.
Pour remaining batter, top with remaining apples + swirl/topping. Use a knife to lightly drag through layers for swirl effect if desired. 
Bake in the preheated oven for about 55-65 minutes, or until a toothpick inserted in the center comes out clean. If top is browning too fast, tent with foil partway through.
Allow the loaf to cool in the pan for a bit (10-15 minutes), then transfer to a wire rack to cool more. Once slightly warm or cool, whisk together glaze (powdered sugar + milk + vanilla) and drizzle over the top.