Disclaimer: I am one of the core developers of youtube-dl.
What you see as a unified YouTube is in reality a lot of different protocols and variations. So while every video looks the same, there is considerable variation:
- For some videos, the API is quite stable and uses HTTP. This class seems to be picked for most "ordinary" videos uploaded by you and me and not some large organization. In recent times, they've switched more and more towards DASH, although the legacy format remains for low-quality (<1080p) videos.
- Music videos typically use HTTP or DASH, but require an extra step of decoding the URL, which can be quite complicated since it involves interpreting Flash or JavaScript code.
- Movies seem to use yet another wholly different scheme, namely RTMP.
- Live streams, even when recorded, typically use HLS, sometimes exclusively.
Of course, there is considerable overlap, many videos are available in multiple formats out of HTTP, HTTP with signature, DASH, DASH with signature, RTMP, HLS, HLS with signature. Without a public statement by YouTube, all we can do is guess and implement everything - and that's precisely what the youtube-dl project (and probably the other projects you listed) tries to do.