Question When a player refunds to GooglePlay or AppStore after an in-app purchase

I use Unity's IAP
I use Firebase to let players use email to create account passwords. When a player refunds to GooglePlay or AppStore after an in-app purchase, how do I know which account has refunded and how do I receive the refund message? I want to take back a sold item

 

If you are not saving purchases locally (for example in PlayerPrefs) but only access the received purchases from Unity IAP on each app launch, after a refund the purchase will be gone in that list.

If you save purchases to a player's account on Firebase too, you obviously need to use server-server notifications and map the received transaction ID to your player account on Firebase. This is not an easy task and requires some database design and server or REST programming.

 

I'm looking for teaching, I don't know if there are keywords to search for, thank you