what are “in app “purchases and how can you get the price of an app before hitting get?

in app purchases are literally just that, a purchase within the app itself, so say a game like fortnite might be free to use, but some extras ( which aren't mandatory to own) but you might want them, such as weapons, clothing , etc will require in app purchases.

or maybe photo editors that are free to use on a basic level but offer some pro features for an in app purchase.

The prices are usually shown either along with the listed items for purchase, or at the checkout .... the prices are shown before you buy usually

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