Does Netsuite have any function that would be able to join Saved Search tables (i.e. JOIN Sales Order, Return Authorization ON PO #)?
Saved searches are limited to one level of join. If you need multiple levels you can use scripting to achieve multiple joins by retrieving datasets using separate saved searches and merging those using common fields.
You could just use SuiteAnalytics in the UI, it has 5 levels of joins.
To answer the main question, in the UI at the bottom of the list of things in criteria/columns, there are things that say fields..., those are all joins.