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NewPipe doesn't use the API, it scrapes everything from the website. And it includes its own youtube-dl-like library.

Newpipe does not use Youtube APIs.

https://newpipe.schabi.org/FAQ/

Yes, it has live playback with mpv and you can even use it on Livestreams.

And it can also get around the age block without logging in...

The best part is being able to play videos without the bloat of JavaScript

Ytdl is use by other programs as a backend and can so indirectly support streaming rather than downloading; ex. if you have both installed, you can `mpv $YT_URL` and it'll work.