You can choose which fields to be previewed on the portal layout (e.g. grid/gallery) or linked records on the form. There are 2 ways:
1. Select Custom Fields (for the portal)
There is an option to override the default preview fields by setting up the Custom Fields for Layout. Instead of showing the top fields of the child form, you'll pick a list of fields in this option. This will affect the grid, list, and gallery view.
2. Child Form
Can also be called a nested form or sub-form. It is a form that lives inside the linked records field. You can access it by clicking on this button here:
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 linked record or the grid.
Note that linked records are 'expandable' by default if the linked field has a child form (allows expanding/editing). There's no new option for it in the settings.
⚠️ Heads up
Note that the following field types are skipped and can't be included in the preview:
Linked records fields
Lookup fields that contain linked records
In the List layout, you can hide the title of the fields by turning on the Hide title for preview fields toggle in the View records section.