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.
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.
Eric has been writing about tech for over 27 years. He was on the founding staff of Windows Sources, FamilyPC, and Access Internet Magazine (all defunct, and it's not his fault). He's the author of two novels, BETA TEST ("an unusually lighthearted apocalyptic tale"--Publishers' Weekly) and KALI: THE GHOSTING OF SEPULCHER BAY. He works from his home office in Ithaca, New York. 
There are workarounds. One is using a free iOS file manager app like Documents by Readdle. Use the Safari browser to visit a YouTube video, and use the Share option to find Copy Link. Then go back to Documents, and use the built-in browser in the app to surf to a video download helper site like SaveFrom.net. Paste the link into the form (hold your finger down on it until you get "Paste" menu to pop up). The site will give you the links to download, and you can save the file to Documents. Hold down and drag the file up, until you're back on the main screen, then drag it to the Photos folder. You'll need to give Documents permission to access Photos the first time. You can then access the video like you would any video taken on the phone or tablet—in the Photos app. 
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