I certainly appreciate the work of youtube-dl developers, but (in order to test it out) I was just able to go to the YouTube page, open the network request log on my browser, and pick out the particular URLs (one for video, one for audio) the content is downloaded from, just from the MIME types displayed. Then I was able to replay those requests (using Copy as cURL), removing the range argument, in order to download the video and audio to my machine, in an easily-playable format. (From there, nearly any video conversion software could combine video and audio into one container.)
That's a far cry from having to pick out those segments from the memory of my browser as it runs.
Edit: I may have misread your comment, apologies if I misunderstood.