On each platform (Windows and iOS), the respective App Stores are responsible for handling any queries or issues with your purchase, just as they are for any other app or content you purchase through the App Stores. They handle all the billing/account enquiries and are the only teams that can transfer your license to another account, or refund your purchase.
The Microsoft Store team can be contacted via their standard contact form:
https://support.microsoft.com/en-gb/contactus/
For the Apple App Store, contact Apple Support for general account questions, and there's information about how to apply for a refund here