For the Objective-C/Swift symbols to be accessible by the Nativescript runtimes the following criteria should be met:
1) They need to be compiled and linked
2) Metadata needs to be generated for them
The first task is done by the NativeScript CLI by adding the source files to the generated .xcodeproj. For the second one the Metadata Generator needs to find a module.modulemap of the compiled modules.
Note: For .swift files module.modulemap is not required.
In order to satisfy the above constraints the developer has to:
1) Place the source files in App_Resources/iOS/src/
2) Create a modulemap for the Objective-C files
Note: Swift classes need to be accessible from the Objective-C runtime in order to be used from NativeScript. This can be done by using the @objc attribute or by inheriting NSObject.
You can find a detailed walkthrough on how to use native source code in NativeScript here.