Portals display records based on a relationship. The tables are grayed out because they are not related to the primary table.
Fair enough, but for example, on the Microsoft side, if I want a form to display all records where the status is opened, I would create a query, and link the form to the query as the data source. How do I do that in FileMaker?
Filemaker does it with Tutorial: What are Table Occurrences?, the "boxes" found in Manage | Database | relationships.
You can open that window/tab panel and click your existing table to select it, then click the ++ button to duplicate the table occurrence. This creates a new reference to the same table and now you can define a relationship between two occurrences of the same table. You can use special match fields or a portal filter to limit the records to just those with status= "open".