Interesting. I entered a URL into Signal in 'note to self' (didn't send) and in my access log I saw

    162.210.7.8 - - [20/Jul/2022:16:40:18 -0500] "GET / HTTP/1.1" 200 15336 "-" "WhatsApp/2"

of course, anyone can send any user agent they like. Maybe signal is doing this for better compatibility. I did a quick search in the open source of signal-android and found this: https://github.com/signalapp/Signal-Android/blob/e22ddb8f96f...

  public LinkPreviewRepository() {
    this.client = new OkHttpClient.Builder()
                                  .cache(null)
                                  .addInterceptor(new UserAgentInterceptor("WhatsApp/2"))
                                  .build();
      }

so yes the client app generates a preview of an entered URL by making requests with the user agent WhatsApp/2.