How to access the nested fields in a json using expression language in Nifi?

I would do this with a Groovy based InvokeScriptedProcessor 

Using this code:

import groovy.json.JsonOutput
import groovy.json.JsonSlurper
import java.nio.charset.StandardCharsets
import org.apache.commons.io.IOUtils

class GroovyProcessor implements Processor {
    PropertyDescriptor BATCH_SIZE = new PropertyDescriptor.Builder()
        .name("BATCH_SIZE")
        .displayName("Batch Size")
        .description("The number of incoming FlowFiles to process in a single execution of this processor.")
        .required(true)
        .defaultValue("1000")
        .addValidator(StandardValidators.POSITIVE_INTEGER_VALIDATOR)
        .build()

    Relationship REL_SUCCESS = new Relationship.Builder()
        .name("success")
        .description('FlowFiles that were successfully processed are routed here')
        .build()

    Relationship REL_FAILURE = new Relationship.Builder()
        .name("failure")
        .description('FlowFiles that were not successfully processed are routed here')
        .build()
    
    ComponentLog log 
    
    void initialize(ProcessorInitializationContext context) { log = context.logger }
    Set<Relationship> getRelationships() { return [REL_FAILURE, REL_SUCCESS] as Set }
    Collection<ValidationResult> validate(ValidationContext context) { null }
    PropertyDescriptor getPropertyDescriptor(String name) { null }
    void onPropertyModified(PropertyDescriptor descriptor, String oldValue, String newValue) { }
    List<PropertyDescriptor> getPropertyDescriptors() { Collections.unmodifiableList([BATCH_SIZE]) as List<PropertyDescriptor> }
    String getIdentifier() { null }

    JsonSlurper jsonSlurper = new JsonSlurper()
    JsonOutput jsonOutput = new JsonOutput()
    
    void onTrigger(ProcessContext context, ProcessSessionFactory sessionFactory) throws ProcessException {
        ProcessSession session = sessionFactory.createSession()
        try {
            List<FlowFile> flowFiles = session.get(context.getProperty(BATCH_SIZE).asInteger())
            if (!flowFiles) return

            flowFiles.each { flowFile ->
                List data = null
                session.read(flowFile, { 
                    inputStream -> data = jsonSlurper.parseText(IOUtils.toString(inputStream, StandardCharsets.UTF_8)) 
                } as InputStreamCallback)
                List outputData = []
                data.each { order ->
                    outputData.add("${order.orderId} ${order.orderName}")
                    order.orderItems.each { orderItem ->
                        outputData.add("${orderItem.orderItemId} ${orderItem.orderItemName}")
                    }
                }
                FlowFile newFlowFile = session.create()
                newFlowFile = session.write(newFlowFile, { outputStream -> outputStream.write(outputData.join('\n').getBytes(StandardCharsets.UTF_8)) } as OutputStreamCallback)
                session.transfer(newFlowFile, REL_SUCCESS)
                session.remove(flowFile)
            }
            session.commit()
        } catch (final Throwable t) {
            log.error('{} failed to process due to {}; rolling back session', [this, t] as Object[])
            session.rollback(true)
            throw t
        }
    }
}

processor = new GroovyProcessor()

Don't let all that code scare you when the part that's doing the formatting is only these lines:

This is the generated output: