Updated comments which looked outdated to me.

The example here:
https://developers.google.com/identity/sign-in/web/backend-auth
Shows:
new GoogleIdTokenVerifier.Builder(transport, jsonFactory)
But doesn't show where the transport or jsonFactory come from.

It was easy to find the right JSON library from the left-nav here:
https://developers.google.com/api-client-library/java/google-http-java-client/setup

But it was not so easy to find a HTTPTransport from that page or from the Javadoc here:
https://github.com/google/google-http-java-client/blob/dev/google-http-client/src/main/java/com/google/api/client/http/HttpTransport.java

Both those sources suggested that plain old Java projects use com.google.api.client.javanet.NetHttpTransport which I could not find.  But com.google.api.client.googleapis.javanet.GoogleNetHttpTransport was already included in the google-api-cient and worked for me!

The suggestion to use GoogleNetHttpTransport came from here:
https://developers.google.com/google-apps/tasks/quickstart/java#step_3_configure_the_project_build

I hope this is helpful.

google-http-client/src/main/java/com/google/api/client/http/HttpTransport.java

@@ -52,8 +52,10 @@
  * </li>
  * <li>Other Java environments
  * <ul>
+ * <li>{@code com.google.api.client.googleapis.javanet.GoogleNetHttpTransport} is included in
+ * google-api-cient 1.22.0, so easy to include.</li>
  * <li>{@code com.google.api.client.javanet.NetHttpTransport} is based on the HttpURLConnection
- * built into the Java SDK, so it is normally the preferred choice.</li>
+ * built into the Java SDK, so it used to be the preferred choice.</li>
  * <li>{@code com.google.api.client.apache.ApacheHttpTransport} is a good choice for users of the
  * Apache HTTP Client, especially if you need some of the configuration options available in that
  * library.</li>