Recently discovered this. Molly supports link with existing device just like on signal desktop. It even has benefit of getting entire chat history unlike signal desktop. Just restore the signal backup file during setup and then click link with existing device. Then scan with you primary phone. Beauty of open source. Molly: https://molly.im/
The server could decrypt or could be machines attached to the server that store data
Your private key stays at the client, the server doesn't get it. Verifiable by the source code of your client
Ook thanks