>This is of note, I think, because the TV client is a way to get high-quality video from the YouTube API without having to pass it a valid YouTube login token
Are you talking about the "1080p premium" quality tier that you normally have to pay to get?
Low trust as in intended to be used by 3rd parties without Javascript or any form of attestation. Like the Wii U client or I think the iframe embeds at one point.
(maybe they've all been killed by now, I haven't been paying too much attention...)
Then what's the "high quality" video? Anyone can use the web interface of youtube and watch it without any DRM (for now). Why are they so jellously guarding one specific API when the others are wide open?
It's the VP9 high bitrate codec, to get it you need the cookie from a premium subscribed account or yt-dlp can get it by default by emulating an iPhone header if I recall correctly.
--extractor-args "youtube:player_client=default,ios"
But this is done by default.
As somewhat related know that there's also a higher bitrate OPUS codec that's only available to premium cookies, it would then be the default YouTube music codec.
Are you talking about the "1080p premium" quality tier that you normally have to pay to get?