I recommend reviewing the persistence docs, the InfluxDB+Grafana tutorial and the Persistence with Groups Design Pattern.
In OH, persistence saves the states of Items. There is a config that specifies what database is used, which Items are saved, and how to save the Item's states (e.g. every change, every update, every minute, etc.).
But be aware that one does not interact directly with the database. One stores data into the database by changing or updating the state of the Item. One retrieves data from the database by calling one of the persistence methods on an Item.
In short, you can only work with Items. If you need to get at data that isn't represented by an Item you must use some other way to get at the data, such as calling the REST API.