Hello! Great question. I think it was touched on in the response in French but basically tarte de pommes would mean the pie was MADE of apples whereas it actually just contains apples. Think apple juice - jus de pomme. It’s actually made of apples.

pasque la tarte c'est avec les pommes, sa tarte aux pommes, bien sûr, mais la raison pour laquelle sa tarte aux pommes est parce qu'elle contient des pommes, sans les pommes, ce ne serait que de la tarte. donc, c'est une tarte aux pommes.

I’m going to answer in English because I teach French in the U.S. and am used to explaining grammar in English. Both prepositions are possible, but there is a slight difference in meaning. Using « à » plus one or more other words as a modifier indicates an attribute of an object, or in other contexts, a purpose, while using « de » plus one or words as a modifier indicates composition or content. You can see that the difference is very small. In this specific case, « tarte aux pommes » indicates the type of pie, while « tarte de pommes » says that the pie is made of apples. Obviously, both are true here, but « tarte aux pommes » happens to be a far more common expression. Hope that makes sense…

Trying to directly translate prepositions is usually a fruitless activity. Often completely different prepositions are used to express similar concepts even in closely related languages. It's best to just learn the collocations in your target language rather than spend time wondering why they don't use the same prepositions that you're accustomed to in your native language.