You are right. Both partners needs to be online at the same time. That's why the sender will provoke a push that will wake up the recipient when it becomes online. This recipient will in turn cause a push that will wake the sender and then the connection will be established. Usage of WebRTC is explained here: https://www.skred.app/skred-peer-to-peer-secure-communications-using-webrtc/
Comparing to SimpleX, Skred is operating without servers for exchanging messages via the WebRTC protocol. A signaling server is used to send pushes to devices which will be able to established a peer-to-peer connection and do their stuffs ^^
Skred is supported by the french radio "Skyrock" (who also created Skyblog on 2002, a free-speech blog platform, closed recently). We developed Skred as a secure messaging app to provide a sovereign service.
You are right. Both partners needs to be online at the same time. That's why the sender will provoke a push that will wake up the recipient when it becomes online. This recipient will in turn cause a push that will wake the sender and then the connection will be established. Usage of WebRTC is explained here: https://www.skred.app/skred-peer-to-peer-secure-communications-using-webrtc/