Pretty much. Signals servers just initiate connection between clients, who in turn negotiate parameters with each other so that messages can be encrypted and sent privately between the clients. The messages never has to touch Signals servers, unless you’ve turned on certain features. It’s what you could call a “peer-to-peer chat”.
It’s apps for Android and iOS are available on GitHub, as well as libraries dedicated to the Signal protocol that you can use and implement in your own projects.
So it’s transparent, private and secure. Pretty boss. Waiting for someone to correct me on this one ^^;