A QR code generator app is built using FastAPI, a modern web framework for building APIs with Python. The app generates QR codes from provided URLs and allows users to add custom watermarks to the QR codes. The app validates that the URL field is provided in the request. It uses the qrcode library to create QR codes and PIL to resize and paste the watermark images. The app saves the generated QR code image with a filename that includes the URL, allowing users to download or use the QR code.