Very simplified: Client sends a video stream to zoom. Zoom reencodes the video on the fly in different formats and forward to the 10 participants separately.

Very simplified: Client sends a video stream to zoom. Zoom reencodes the video on the fly in different formats and forward to the 10 participants separately. Participants have different devices and network connectivity, that's how the dirty real world is, so zoom has to do that. It's development work and it's compute intensive but it's a hard requirement to have "good" videoconferencing. Imagine the same thing with E2E. Client sends a video steam to zoom. Zoom can't do shit with it because it's encrypted, it can't be decoded it can't be reencoded.