Square app.

Square is a whole lot more than the user app itself. There is a whole lot of backend merchant processing (and even more security mechanics) that is involved.

But consider the actual user frontend application.

If you were to code that in Xcode you would likely use a Master-Detail template to construct the app. You would have a nav bar helping you to change between your different views, and a table that would display transactions.

On the backend you would use the framework that would allow you to interact with the merchant services of an established provider, or perhaps you would become your own merchant provider (very very complex and lots of legal stuff involved).

But, and here is the crunch, you need to read the credit cards. You would need to develop an accessory and gain approval from apple to use it with the device. This would involve writing some low level code that would interact with the lightning port or the audio port so that your card reader accessory could be used.

Finally, you'd need to market it. Remember, most established merchant services are quite trusted and secure, and the market is already quite saturated. To partner with these existing providers would take a fair amount of your profit margin per transaction making it difficult for you to have enough margin to sustain your product development and support.

In short, perhaps not a profitable business model given the challenges and the market.