The implementation for the JVM looks like this:

internal actual fun handleCoroutineExceptionImpl(
        context: CoroutineContext, 
        exception: Throwable) {
       
    ServiceLoader.load(CoroutineExceptionHandler::class.java)
        .forEach { handler ->
            handler.handleException(context, exception)
        }
    val currentThread = Thread.currentThread()
    
    currentThread.uncaughtExceptionHandler
        .uncaughtException(currentThread, exception)
}

It looks for exception handlers using the ServiceLoader and forwards the exception to all the ones it can find. It also forwards the exception to the handler of the current thread.