App Store: Open the App Store, tap your profile and choose Purchase History. You can also check purchases at Apple’s Report a Problem service.
Google Play: Open your profile, then Payments & subscriptions → Budget & history, or find the purchase-confirmation email.
Website: Sign in to the account used for the purchase and check its billing or invoice section. Contact support if you cannot locate the document.
Make sure you are checking the purchasing account. Receipts and tax-invoice options depend on the billing provider.