Correct me if im wrong, but if you play a 1080p video on a 4k screen, that would be upscaled. If you put a 1080p video in a 4K stream, then play that 4k stream on the 4k screen, no post-processing would be applied to the video on the screen. All the upscaling happens during encoding, where you have far more control over the upscaler quality.

Just use yt-dlp instead of relying on websites that shove ads in your face and may do what ever they want to the files you’re downloading?