ObjectMacros.h has the following macro definition:
// Declare that objects of class being defined reside within objects of the specified class.
#define DECLARE_WITHIN_INTERNAL( TWithinClass, bCanUseOnCDO ) \
/** The required type of this object's outer ({{ typedef-type }}) */ \
typedef class TWithinClass WithinClass; \
TWithinClass* GetOuter##TWithinClass() const { return (ensure(bCanUseOnCDO || !HasAnyFlags(RF_ClassDefaultObject)) ? (TWithinClass*)GetOuter() : nullptr); }
I assume that’s where the specific GetOuterX() function is defined for the object specified via Within=X in the UCLASS