> But the author also mentioned the screenshot approach before using Passkit and called it the naive approach, so it’s likely they did something considerably simpler.

I guess I didn't realize it was that simple a question.

The "how" there is "take a screenshot of the first-party app". In many cases (especially with physical barcodes like a loyalty card you can just photograph), that's all you need; just keep it in your photo roll. It didn't work in this case because the QR codes contain some sort of signature or expiration date that prevents a screenshot from last week from working this week.

If it has to be in wallet, https://developer.apple.com/documentation/passkit/pkpasstype....