I am using sever to server notifications of ios https://developer.apple.com/documentation/storekit/in-app_purchase/enabling_server-to-server_notifications. It is giving me the notifications but there are no information that to which user this transaction belongs or who paid for it, so am I missing something or there are other ways to know?
You can only rely on original_transaction_id. It's unique identifier of subscription. In case you have one subscription group it also identifies your user. However you have to implement your own logic by matching your users with original_transaction_ids.
You can also use ready-to-use solution to handle Apple notifications, like Apphud.