public static <K, V1, V2, VR> StreamStreamJoinNodeBuilder<K, V1, V2, VR> streamStreamJoinNodeBuilder() {
        return new StreamStreamJoinNodeBuilder<>();
    }
Too much information to generalize, so Stream-Stream joins are represented by a specific node.
streams/src/main/java/org/apache/kafka/streams/kstream/internals/graph/StreamStreamJoinNode.java