extension_attributes.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
    <extension_attributes for="Magento\TestModule1\Service\V1\Entity\Item">
        <attribute code="custom_attribute_data_object" type="Magento\TestModuleMSC\Model\Data\CustomAttributeDataObject" />
        <attribute code="custom_attribute_string" type="string" />
    </extension_attributes>
    <extension_attributes for="Magento\TestModuleMSC\Model\Data\CustomAttributeDataObject">
        <attribute code="custom_attribute_nested" type="Magento\TestModuleMSC\Model\Data\CustomAttributeNestedDataObject" />
        <attribute code="custom_attribute_int" type="int" />
    </extension_attributes>
</config>