Ambari-infra is SOLR but it used internally by HDP services. Think about where Ranger, Atlas and other services are storing/indexing their data which you are able to search in sub seconds. This is all done in Ambari-Inra which is SOLR.
Ambari-infra is NOT where our customers can store and index their own data. If you have a SOLR use case, you cannot use Ambari-infra. It is not supported. If you want to use SOLR for indexing your data, then you install a separate SOLR service using Ambari (HDP 2.5 and future versions). Link here.