<virtualType name="Magento\Framework\Setup\Declaration\Schema\Config\SchemaLocator"
             type="Magento\Framework\Config\SchemaLocator">
    <arguments>
        <argument name="realPath" xsi:type="string">
            urn:magento:framework:Setup/Declaration/Schema/etc/schema.xsd
        </argument>
    </arguments>
</virtualType>

This is a virtual type configuration.

This configuration is the application level configuration (`app/etc/di.xml`) fo the object manager. It is not a part of a specific module, but will apply to all sub-applications in Magento 2 (cart, admin, api, cron, etc).

This virtualType configuration tells the Object Manager when a user wants a virtual Magento\Framework\Setup\Declaration\Schema\Config\SchemaLocator object that it should return a Magento\Framework\Config\SchemaLocator object.

This configuration tells the Object Manager that when a user wants a Magento\Framework\Setup\Declaration\Schema\Config\SchemaLocator object that the following arguments should be used in the foo object's constructor.

• $realPath = "urn:magento:framework:Setup/Declaration/Schema/etc/schema.xsd";

<virtualType name="Magento\Framework\Setup\Declaration\Schema\FileSystem\XmlReader"
             type="Magento\Framework\Config\Reader\Filesystem">
    <arguments>
        <argument name="fileResolver" xsi:type="object">
            Magento\Framework\Config\FileResolverByModule
        </argument>
        <argument name="converter" xsi:type="object">
            Magento\Framework\Setup\Declaration\Schema\Config\Converter
        </argument>
        <argument name="schemaLocator" xsi:type="object">
            Magento\Framework\Setup\Declaration\Schema\Config\SchemaLocator
        </argument>
        <argument name="fileName" xsi:type="string">
            db_schema.xml
        </argument>
        <argument name="idAttributes" xsi:type="array">
            <item name="/schema/table" xsi:type="string">
                name
            </item>
            <item name="/schema/table/column" xsi:type="string">
                name
            </item>
            <item name="/schema/table/constraint" xsi:type="string">
                referenceId
            </item>
            <item name="/schema/table/index" xsi:type="string">
                referenceId
            </item>
            <item name="/schema/table/index/column" xsi:type="string">
                name
            </item>
            <item name="/schema/table/constraint/column"
                  xsi:type="string">
                name
            </item>
        </argument>
    </arguments>
</virtualType>

This is a virtual type configuration.

This configuration is the application level configuration (`app/etc/di.xml`) fo the object manager. It is not a part of a specific module, but will apply to all sub-applications in Magento 2 (cart, admin, api, cron, etc).

This virtualType configuration tells the Object Manager when a user wants a virtual Magento\Framework\Setup\Declaration\Schema\FileSystem\XmlReader 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\Framework\Setup\Declaration\Schema\FileSystem\XmlReader object that the following arguments should be used in the foo object's constructor.

• $fileResolver = new Magento\Framework\Config\FileResolverByModule (...)
• $converter = new Magento\Framework\Setup\Declaration\Schema\Config\Converter (...)
• $schemaLocator = new Magento\Framework\Setup\Declaration\Schema\Config\SchemaLocator (...)
• $fileName = "db_schema.xml";
• $idAttributes = [...items...];