How should I learn to build a simple digital loyalty card app from scratch?

This is a good use case for a progressive web app rather than a native mobile app. Learn the basics of a web framework, authentication, and a database, then build a simple stamp ledger. The core logic is fairly straightforward once you understand how users and transactions are stored securely.

Start with a deliberately small MVP instead of trying to build a complete retail platform. You mainly need customer sign-in or identification, a merchant dashboard, and a server-side database that records each stamp. Once that works reliably, you can add QR codes, wallet passes, notifications, or more advanced rewards.

The difficult part is really identifying the customer, not designing the card screen. Staff should be able to find someone in a couple of seconds without requiring an app installation. A phone number, customer code, or printed QR key tag could work. Keep the stamp count on the server, not only in the browser, or people could easily alter it.

Think of the loyalty feature as part of a basic sales and customer system. You need customers, purchases, and a way to associate each purchase with a customer before rewards can be calculated. Begin by recording a sale and awarding a stamp, then add reward rules such as giving a discount after a certain number of visits. That approach will give you a much stronger foundation than starting with the visual loyalty card alone.