The main difference between 720p upscaled to 1080p and native 1080p lies in the underlying image quality and resolution. 720p video has a resolution of 1280 x 720 pixels, while 1080p video has a resolution of 1920 x 1080 pixels. This means that native 1080p video has significantly more pixels, resulting in a sharper, more detailed image.
When 720p video is upscaled to 1080p, the image quality does not match that of true 1080p video. Upscaling a lower resolution image to a higher resolution inevitably introduces some quality loss and artifacts. The upscaling process uses algorithms to interpolate and estimate the missing pixels, but this cannot fully recreate the clarity and fine details that a native 1080p source would provide. While upscaling can make a 720p image appear larger and fill a 1080p screen, it cannot match the inherent visual quality of video that was originally captured and encoded at the 1080p resolution.