Normally in modern adaptive streaming, every video variant is muxed into a separate stream without audio, and different audio variants are muxed into their own individual streams.

wow, I wonder if that's why it always feels so frequent an experience of mine where the audio and video feel subtly out of sync with one another. It's very minute but detectable. Feels like that experience has increased in the last 6 months or so.

YouTube has been that way (separate streams) for a long time, definitely not anything new in the last 6 months. And they reassemble to be indistinguishable from the original combined stream, so that's not going to be the cause.

There are plenty of causes of delayed audio, however. Bluetooth is a big one, if your device and software aren't properly compensating for the Bluetooth transmission delay.

That kind of issue on the encoder OR player side would've been easily caught in testing, including through automated tests, so as crazygringo says, it's most likely your OS/hardware.

If you need to troubleshoot sync issues, this is a helpful tool: https://www.youtube.com/watch?v=HD4emXqHCsE

that's pretty good, bip bop, those line up spot on. I wonder if its quality/resolution dependent. That test video is 360 max. Whatever I'm perceiving if I'm actually seeing something is in the ms range, it's not obvious it's like right on the border if I trust my senses.

Only some formats which I guess are to be used with older browsers contain both video and audio. In general, these days video and audio are delivered through separate streams on YouTube.

grab the audio stream from something else and stitch them together with ffmpeg (like youtube-dl and others do)