<virtualType name="Magento\Elasticsearch\Model\Adapter\Index\Config\Reader"
             type="Magento\Framework\Config\Reader\Filesystem">
    <arguments>
        <argument name="converter" xsi:type="object">
            Magento\Elasticsearch\Model\Adapter\Index\Config\Converter
        </argument>
        <argument name="schemaLocator" xsi:type="object">
            Magento\Elasticsearch\Model\Adapter\Index\Config\SchemaLocator
        </argument>
        <argument name="fileName" xsi:type="string">
            esconfig.xml
        </argument>
    </arguments>
</virtualType>

This is a virtual type configuration.

This configuration is for the base area. Base area configurations will apply to all the sub-applications.

This virtualType configuration tells the Object Manager when a user wants a virtual Magento\Elasticsearch\Model\Adapter\Index\Config\Reader object that it should return a Magento\Framework\Config\Reader\Filesystem object.

This configuration tells the Object Manager that when a user wants a Magento\Elasticsearch\Model\Adapter\Index\Config\Reader object that the following arguments should be used in the foo object's constructor.

• $converter = new Magento\Elasticsearch\Model\Adapter\Index\Config\Converter (...)
• $schemaLocator = new Magento\Elasticsearch\Model\Adapter\Index\Config\SchemaLocator (...)
• $fileName = "esconfig.xml";