When viewing a 1080p video on a 720p monitor, the video quality will be downgraded. The 1080p video will be scaled down to fit the 720p resolution of the monitor, resulting in a lower-quality image.
The primary reason for this is the difference in resolution between the two. A 1080p video has a resolution of 1920 x 1080 pixels, while a 720p monitor has a resolution of 1280 x 720 pixels. When the 1080p video is displayed on the 720p monitor, the image must be scaled down to fit the smaller resolution. This scaling process can result in a loss of detail and sharpness, leading to a lower-quality viewing experience.