Stock Android does not, by inspecting network traffic, contact Google servers.

Google play services and other GApps, do, and they can be exploited in this traffic, or told by Google to activate other backdoors.

Signal with GApps, Google can know which phones, and which users, are using Signal, thats a security vulnerability. Google can infer from their Google-messaging thing, that notifications are sent, and have a high probability of knowing if it is to Signal. Who talks when is leaked to Google.