How can I download youtube video 1080p or 4k fully quality?

Correct, the online youtube video downloaders does not support 1080P high quality videos.

1080P refers to a video resolution with 1920 pixels in width and 1080 pixels in height, commonly known as Full HD (FHD). The "P" in 1080P stands for "progressive scan," meaning each frame of the video is drawn sequentially, providing smoother motion compared to interlaced formats. 1080P is one of the most popular resolutions for high-definition videos and is widely supported by modern screens, from monitors to televisions.

Why Don't Online Downloaders Support 1080P from YouTube Videos?

Online downloaders may not support downloading YouTube videos in 1080P due to several reasons:

Video Encoding: YouTube often separates video and audio streams for higher-quality videos. 1080P videos may have their video and audio stored in separate files (video in one format and audio in another), and online downloaders may not support downloading both parts together.

Streaming Protocols: YouTube uses adaptive streaming protocols like DASH (Dynamic Adaptive Streaming over HTTP) and HLS (HTTP Live Streaming) to deliver high-definition content, and online downloaders may struggle to process these advanced streaming methods.

For downloading 1080p youtube videos, desktop video downloaders typically provide better support, as they can handle multiple streams and bypass some of the limitations of online services.