In the portal, you can display the data as a list of records.
In the form, the data is displayed in a long vertical card (with linked records displayed as a list).
💡If you want to show "fields" rather than "records" when the user logs in, you should use the form extension. The form can be used as a portal (when the login feature). It allows you to have a list of different fields on the same page (e.g. linked records and rollup field).