You can select which fields to be previewed on the linked records (or portal) by changing the order of the fields in the child form. Whichever fields you move to be in the top 3-6 spots in the child form, they'll be shown on the outside of the record.
It is a form that lives inside the linked records field. It can also be called a nested form or sub-form. You can access it by clicking on this button here,
If the linked form includes an attachment field, the first image will be shown as well.
The 'expanded' linked records feature is enabled by default if the linked field has a form (allows expanding/editing records). There's no new option for it in the settings.
⚠️ Heads up
Note that the following fields are skipped and can't be included in the preview:
Linked records fields
Lookup fields that contain linked records