Value Helps
Every input field can have a value help (also for search fields). A value help is usually based on a CDS view that provides the lookup data.
You connect the field to the value help using the field annotation:
@Consumption.valueHelpDefinition.entity: { Name : <CDS View>,
Element: <FieldName> }
The value-help CDS view can read from any table and can be implemented as a CDS Table Function using SQLScript if needed.
A frequent scenario is using the table DD07T (domain fixed values). You can reference it directly.
More complex value helps are implemented with the additionalBinding where you can bind multiple fields so the UI values restrict the value help.