4K video, with its resolution of 3840 x 2160 pixels, has significantly more visual information than standard 1080p video with a resolution of 1920 x 1080 pixels. When a 4K video is played on a 1080p monitor, the video is downscaled or scaled down to fit the lower resolution display. This downscaling process allows the 1080p monitor to display the richer, higher-quality visual information from the 4K source.
The downscaling of 4K to 1080p is done using advanced algorithms that preserve fine details and minimize pixelation or blurriness. In contrast, a native 1080p video played on a 1080p monitor does not require any downscaling, but may have more compression artifacts and lower visual fidelity compared to the optimized downscaling of the 4K source. This is why 4K videos can often appear sharper, more detailed, and of higher overall quality when viewed on a 1080p monitor, despite the monitor's lower native resolution.