Today I found a video on YouTube that surprised me (https://www.youtube.com/watch?v=Pkc3sE6iKV4). It is by Rob Braxman, a privacy advocate with a lot of expertise.
He talks about all kinds of video conferencing situations. Like one-to-one, one-to-many and many-to-many. It is quite obvious that one-to-many and many-to-many uses a central server as a relay server and therefore no end-to-end encryption is possible.
However, he also says that WebRTC TURN servers (which are used in unfortunate Internet situations such as NAT, firewalls, etc.) cannot provide end-to-end encryption because the server must capture and relay the video blocks. Only encryption during transmission is possible, so that anyone who controls the server can watch the traffic. I thought the TURN relay servers just forward the encrypted data.
The interested part of the video begins around 20min.