I use BlueJeans at work is this is what they do. I'm not so sure that Zoom is doing this. The side effects are that it heavily restricts what kinds of layouts you can use, and you will frequently see everyone's face go pixelated in a way thats super clear it's a single stream. Stuff like BlueJeans will only show you at most 9 users in a grid, even if more than 9 have video on. They will never have rows of 3 and then a row of two users. I don't see this behavior with Zoom but I only use it about once a month for random meetings.

I agree with robszumski. I don't think that it's a single stream being configured server-side.

They may have some good video-compression stuff going on. I haven't bothered to set up a network sniffer to see what is going on, but I suspect that it's a PtP vid connection.

This is because I will occasionally see only one thumbnail bork, and also the thumbnails are constantly switching places to match my screen (which I find maddening).

I think that it was Skype that had the "lowest common denominator" problem, where just one bad connection would bork the whole session. It seems that in Zoom, a single (or multiple) bad connection won't mess up the whole call.

Also, the quality doesn't seem to degrade as more people are added.

That's exactly what they're doing. They are using an MCU architecture where as most others use an SFU architecture. I believe some solutions use a hybrid of both.

I guess it's the client who's sending the video that's adapting.