yt-dlp (and youtube-dl?) downloads https://www.youtube.com/s/player/92f199c8/player_ias.vflset/en_US/base.js and extracts the decryption code used to decrypt a signature used to get access to copyrighted videos. At https://github.com/yt-dlp/yt-dlp/blob/1060f82f899b61a0a1c63df37ecdf6dc2bae50e8/yt_dlp/extractor/youtube.py#L2818 it tries to run the code by looking at the code pattern and matching it to python functions. If this fails, it uses PhantomJS (headless browser) to run the proprietary youtube code at https://github.com/yt-dlp/yt-dlp/blob/1060f82f899b61a0a1c63df37ecdf6dc2bae50e8/yt_dlp/extractor/youtube.py#L2821

I'm not 100% sure about the proprietary js thing and only assume it, it needs further investigation.