Works great! And it's FREE!

And, more important to me, it preserves (at least) the original audio stream.

Works with download managers like FDM (Right-click on "Download" button and choose "Save with Free Download Manager"). FDM stores with filename taken from YouTube title plus, easily removed, prefix and suffix, for instance: y2mate.com - 'actual title'_'youtube ID'_1080p.mp4

For sake of simplicity the YouTube button just opens y2mate.com and fills in the youtube URL. That's fine to me as it provides full service with (almost) no privacy concerns. I'm running uBlock Origin and Adguard with lots of filter lists - no problem. Script blockers like NoScript have to be configured to allow scripting for
y2mate.com
ajax.googleapis.com
bootstrapcdn.com
cloudflare.com,
that's it. No persistent or third party cookies required.

Although one of very few other working extensions (Easy Youtube Video Downloder Express - Pro, I recently paid for) integrates the user interface entirely near the Youtube download target, hence, more convenient - it charges you minimum $10 "Donation" to perform HD downloads. It also does unnecessary audio transcoding and video transcoding which always slightly degrades quality (I hear the difference).
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1 month later
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Recently I encountered problems downloading a 6:30 hour audio book in the first place. At the y2mate.com page, depending on Ad-Blocker and browser settings, after clicking the [Start] button to start conversion, y2mate notified me "Video .. too long for conversion." or simply returnd nothing.

Clicking any of the [Download] buttons beside each Video / Audio media type just started playback in browser window instead of downloading something.

But you can still download the complete video or audio by right-clicking the [Download] button and choose "Save target as..." or copying the link address into a download manager. If an extension is missing just add ".mp4" to the downloaded video filename and ".m4a" to an audio file respectively.

Normally the button starts a script invoking conversion and download. For this very long playing time video (despite it's average file size) "YouTube Converter Button" refused conversion but provided a working link for manual download of the entire stream, which looks like this "https://redirector.googlevideo.com/videoplayback?expire=...&...(+ long data section)".

In case of the audio book example VLC reported a MP4 video (although it plays everything that looks like a video, regardless of specific filename extension):
Video Codec: H264 - MPEG-4 AVC (part 10) (avc1)
Audio Codec: MPEG AAC Audio (mp4a)