The SDXL refiner is not an upscaler, it's a separate model with the same architecture used at the same resolution as the base model that is focussed more on detail and less on large scale generation (you can actually use any SDXL-derived model as a refiner, or none; most community SDXL derivatives use a single model with no refiner and beat the Stability SDXL base/SDXL refiner combination in quality.)