I would like to follow-up on this question, in case someone else stumbles upon the same issue.
For the <NATIVE-DECLARATION> Simulink uses the AUTOSAR platform types, as they are defined in the AUTOSAR standard. This is mentioned in the following documentation page,
Please note that these types are used in the generated code and are in turn defined in the header-file Platform_Types.h.
In case you wish to customize the <NATIVE-DECLARATION> data types, then the RTE generator that is used should provide its own header-file Platform_Types.h with the necessary type definitions based on the specific target, since the header-file Platform_Types.h file that is by default provided in Simulink is a stub file intended only for SIL testing.
I hope that this information helps.