Convoro searches with MySQL out of the box and that is complete on its own. On a large community, an OpenSearch server ranks results better.
Enable the OpenSearch module, then go to Admin → OpenSearch, enter the address, choose it as the backend, save, and press Rebuild now. On a big site use php tools/convoro search:reindex instead — the button does the work while your browser waits, and the browser will give up first.
🚨 Use the Test the connection button. If the server does not answer, search quietly carries on using the database — which is the right behaviour, and also means a wrong address looks exactly like nothing happening.
Nothing about who may read what is stored in the index. Each item records which forum or album it lives in, and permissions are worked out fresh on every search — so making a forum private takes effect immediately rather than waiting for a rebuild.