5K Player also features DLNA server playback so videos you grab can be watched on any devices on your home supporting DLNA network; it supports AirPlay for quick playback to supported devices. Pick a video in the library and you can do a quick conversion to MP4, MP3, or even ACC (an audio format preferred by iOS devices). The player didn't like playing back the overly large 4K file though and experienced buffering issues—VLC didn't have any problem with the same file. Ultimately, there's a lot to like about 5K Player, from the price to the features, especially if you look at them as extras on a downloader. But the interface and playback issues may have you looking elsewhere.
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.
Multi-lingual 4K Video Downloader (4KVD) is frequently updated and features clear download links on the program's website; no ad traps here. The software does what it advertises in a simple interface: grabs videos up to 8K in quality and downloads to plenty of formats. Just copy a YouTube URL and click the Paste Link button to get started. 4KVD will even grab subtitles, entire playlists, and all the videos in a channel to which you subscribe. The sites supported are limited to the big names like YouTube, Vimeo, Facebook, and a few others, but covers most of what you need.