How does youtube-dl work?

That's an interesting question. But I think you have better luck asking in their IRC channel: #youtube-dl on freenode.
If you don't know how to use an IRC client you can try the webchat: http://webchat.freenode.net/?randomnick=1&channels=youtube-dl

Please post here what you find, I'm interested in the answer too.

My bet would be that youtube-dl doesn't download any extra JS. But the only way to know that for sure is reading the source code. And if you don't want to do that then asking the developers is the only option left.

Thanks root_vegetable. I found the python script https://github.com/rg3/youtube-dl/blob/master/youtube_dl/extractor/youtube.py

Does youtube-dl give your IP Address to YouTube when you download videos from there? Or is it something like this:

- your request for the video is sent to youtube-dl servers (if there's such a thing)
- the youtube-dl servers download the webpage from YouTube
- the youtube-dl servers send the webpage to your machine
- your machine extracts the video

This would mean that YouTube doesn't know my IP Address. However, I don't know if this is how it works. Therefore, I use torify to send my youtube-dl request through the Tor network for anonymity purposes.