How did I find my rewards

What rewards? Where are you seeing that there are any rewards for your purchases exactly?

In-App purchases are purchases you make inside an App. Like gems or gold, or other in-game consumables. Said items you purchase would appear within the app or game you bought it in. Are those the rewards you are talking about? Or you referring to something additional to what you are purchasing?

Like you buy 100 gems, and you get 50 gold as an additional "reward"?

Or what exactly are these rewards you are referring to?

What rewards exactly?

If you mean the "Daily Cash", it is stored in your Apple Cash card: link-> Get unlimited Daily Cash with Apple Card - Apple Support

What rewards would those be? In App purchases are things you buy inside an App. You get the things you buy. Any additional rewards from those purchases would also be inside the specific App.

Link-> Buy additional app features with in-app purchases and subscriptions - Apple Support