ORTC is what Microsoft has chosen to implement in Edge, so if you use WebRTC in Edge, you're most likely using the WebRTC-over-ORTC adaptor.

In short ORTC is: break WebRTC into objects you manipulate directly, instead of everything being instanciated via RTCPeerConnection. There is significant overlap between the two specs, for instance RTCRtpReceiver / RTCRtpSender.