The RadComboBox exposes the ItemTemplate and SelectionBoxTemplate properties which allow you to define a custom datatemplate for its drop down items and for its selected item when the RadComboBox is not editable.
If you want RadComboBox to visualize more information, you can define a custom DataTemplate and set it to the ItemTemplate property of the control. This template will be applied to the selected item(when RadComboBox is not editable), as well as, all of the items displayed in the dropdown. For an example implementation, check out the Define a Custom ItemTemplate section of the Binding to Collection article in our documentation.
In order to define a separate template for the selected item of a Non-Editable RadComboBox, you can set the SelectionBoxTemplate property of the control to a custom datatemplate. For an example implementation, check out the SelectionBoxTemplate article in our documentation.