Signal choosing not to federate isn't in the scope of this thread. IMO not federating is a guarantee of client quality. The last time I had a look at Matrix E2EE implementations, only Riot was barely usable. There was a ton of clients that didn't support it, didn't intend to, didn't have the confidence to go about implementing etc.
>Also, what about the server source code?
https://github.com/signalapp/Signal-Server There you go, last commit two days ago.