It's probably a simple Gemini query with search grounding or something similar. Not very complicated. You can go onto Google and test it yourself. "Give me the price for Coca Cola for all nearby Checkers, Pick n Pay and Spar":
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You set the Gemini query based on whatever filters your user selects, parse the results into your app, and presto. No need for all the omg where's he getting the data, does he have permission, is he using public apis etc.
It's not cheap though so he probably found a cheaper way to scrape, or he's making money off the app somehow...