URL:
https://www.youtube.com/watch?v=sJsSMT0GEiY
> considering the medium.webm video downloads, once I get a 6 Mb file, the other time a 12 Mb file. Why?
There are both "Dash" & Non-dash varieties of the clips.
Dash have audio & video separately, & if you wanted both, you would have to download both. And if you wanted them to play "together" (as in the clip playing both the audio & video) you would have to join ("mux") them together.
Non-dash are a single clip containing both the audio & video.
Non-dash, webm, medium, is 12 MB, containing audio & video in the one clip.
Dash, video/webm, medium, is 6 MB, containing only the video.
If you also wanted the audio, you would have to download, separately, audio/webm, medium, 3 MB.
And if you wanted the to play "together", like the non-dash, you would have to mux them together.
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