<virtualType name="Magento\Theme\Model\ResourceModel\Design\Config\Grid\Collection"
             type="Magento\Framework\View\Element\UiComponent\DataProvider\SearchResult">
    <arguments>
        <argument name="mainTable" xsi:type="string">
            design_config_grid_flat
        </argument>
        <argument name="resourceModel" xsi:type="string">
            Magento\Theme\Model\ResourceModel\Design\Config
        </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\Theme\Model\ResourceModel\Design\Config\Grid\Collection object that it should return a Magento\Framework\View\Element\UiComponent\DataProvider\SearchResult object.

This configuration tells the Object Manager that when a user wants a Magento\Theme\Model\ResourceModel\Design\Config\Grid\Collection object that the following arguments should be used in the foo object's constructor.

• $mainTable = "design_config_grid_flat";
• $resourceModel = "Magento\Theme\Model\ResourceModel\Design\Config";