If it's not clearly advertised on the front page, _emphasized_ and not a foot note, then it's NOT e2e encrypted.
Example: https://signal.org
For group calls, it depends on how it's implemented, but many group calls are implemented using what's called a Selective Forwarding Unit (SFU). One benefit of SFUs is that they take much less processing for the server than the other kinds (where the video is re-encoded by the server). For those types of group calls, e2e encryption can be implemented with negligible increases to processing and bandwidth.
However, you are correct that it is harder to implement correctly. And it does prevent certain features to be added to the product, such as recording and server-based processing of information (for example, meeting transcriptions).
(I used to work at Google on WebRTC, Duo, and Hangouts, but now work on video calling at Signal).
A OTP might be mathematically simple, but logistically it's very hard - you have to safely distribute the key and that key must be at least as long as the message you're passing.