The results of each Segment is stored in its own table in the Pega Marketing (ExternalMKTData) schema. The the name of the table will be MKTSEG.
The table contains only one column - customerid.
There is no other table created for a Segment.
When you click 'Show Customers' button within the Segment, the additional customer information comes from the backing Report Definition that is run at that moment (when you click 'Show Customers' button).
At 'Run-time' can mean two things: When a Segment run is triggered by other Segment where it is referenced OR when it is triggered from a Program run when the 'Refresh Starting Population' option is enabled. In either cases, the additional customer columns are not involved.