Search API for searching in existing Solr Core
The Solr core's schema must have a field "index_id" and the value must be the same as the machine name of the Drupal index (in order for that (Drupal) index to find it). Moreover, the (Drupal) server has to use "Any Schema" and the (Drupal) index must be read-only.

Note that under "View" it still says "0/0 Indexed", however the search works.

There is a datasource.md in the search_api_solr module's folder which was helpful. I found it helpful to use search_api_devel as it shows the queries to Solr.