In this article, we show in practice how a complete finance app was built—from the home screen to invoice importing and detailed bill management—using Microsoft Copilot as the central development interface, connected to multiple generative AI models. You'll see the exact prompt used for each screen, understand the reasons for choosing each model (Claude 3.7 Sonnet, GPT-4o, Gemini 2.5 Pro, and ChatGPT), as well as the real challenges faced and how each issue was solved.