Does anyone know how this could technically be possible? For a meeting with all Zoom clients they say they use E2E. When a new participant joins, they are immediately added to the meeting.

Is a new pubkey key generated and passed to existing participants? Is there one shared symmetric key that is sent to the new participant? What stops zoom from "adding a participant" and allowing themselves to decrypt the meeting? Is there no server-side transcoding done at all?

It's obvious phone enabled calls can’t be E2E