The same as you do to download any other videos. You just need to choose the 1080p or 4k format in the video result section, sometimes it is in the "video only" section that without sound, then you have to download the audio file (M4A) in the same result section, then you can load the video file and audio files together on your computer using the video playing software, such as VLC. You also can combine the video and audio files by using ffmpeg. Some youtube video does not have HD format originally, in such case, you definitely can not download any HD format video.
If you're willing to do more about the offline YouTube videos, you can try the its advanced version - MacX Video Converter Pro. Comparing with MacX YouTube Downloader, It not only allows you to download YouTube SD/HD/4K videos on Mac, but also convert them to all general-purpose formats, and common devices profiles, edit the downloaded YouTube videos, record screen videos from both desktop and iPhone, or capture videos with webcam. Besides, if you have DVD sources, you can also take advantages of this program to rip DVD to any formats and devices, or make a backup of your DVD movies without any quality loss.
We all know that we should back up our video files on a regular basis and how disruptive and even devastating it can be to lose our entire library of video content. Even with the best of intentions, hard drives can break or we can fall out of the routine of saving our footage in the cloud. Luckily, any video files uploaded in the past to YouTube can easily be salvaged by the channel owner as the site keeps a copy of all your uploaded video content. There are two ways of retrieving these files either #1) go to video manager and download the MP4 although you can only download 2 videos per hour this way or #2) download the entire archive. We show you both options in this week’s Creators Tip.

You just need to copy page URL, or right click on the video to select "Copy video address", then paste the URL into the search box of Distill Video (http://distillvideo.com) and press “Start” button. After waiting 2-3 seconds, the video section showed, choose your preferred format and right click the "Download" button, then click "Save link as..." to download the video or audio into your computer or phone.

Step 3: Choose your targeted video format from the top right corner. There are a lot of formats available. Including MP4, AVI, MKV, and MOV. After this, click on the conversion icon on the right of the chosen file. If you have to convert all the videos in the chosen list, simply choose the “Convert All” option on the bottom right of the main window. 
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