It doesn’t.
Yes, apple fruit does contain acid. Yes, it’s a little bit sour too. I’m not sure about the pH level but if you’re asking because you want to know whether or not to use apple as a substitute for vinegar then yes, it acts as an acidic agent in recipes and the recipe would need to be adjusted accordingly. But apples will never make anything taste sour except applesauce which has all sorts of sugars and flavorings added in order to make them sweet-tangy-cinnamony delicious…
