Which RAP object can be used to organize the display of fields in an app?

Comprehensive and Detailed Explanation From Exact Extract:

UI layout is defined with @UI annotations; it is recommended to maintain the UI annotations in metadata extensions to separate concerns and keep CDS clean.

Metadata extensions provide CDS annotations (such as @UI) outside the data definition and allow layered, switchable UI metadata.

In the RAP tutorial, the “Adding UI Metadata to the Data Model” section explicitly instructs creating metadata extensions for projection views to control the presentation (line items, identification, hidden fields, search, text arrangement). Therefore, the RAP artifact used to organize how fields are displayed in the app is the Metadata Extension.