The primary field in Zenkit is always a text field. It is used as the 'name' or 'title' of a card - the title shown at the top of an item in detail view, and as the main field shown on items in kanban, list and calendar view. You can set any text field as the primary field. Select the text field you'd like to use, then click on the field name in table or detail view. Choose "Set as primary" from the dropdown menu.
You can move the primary field column in table view just as you can with other fields. This doesn't affect how the primary field is displayed in other views. The primary field is also the field used to describe an item in other collections - when an item is referenced in another collection, the primary field is the name given to that item in the collection, and is how you can search for the item to add.