THAT is not the server, its like saying Firefox is opensource so Facebook is too, thanks and no thanks for the link's.

The encryption takes place in the client though, which you can verify by looking at the client source code. I find your comparison with Facebook a bit lacking, a better one would have been by looking at the Firefox code to verify if https traffic is encrypted.

You can setup your own server with Signal, but you will not participate in the same network, so this is not really relevant.

For Country's or big-business it is relevant