magento-test/dev/tests/api-functional/_files/Magento/TestModuleOverrideConfig/etc/config.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:module:Magento_Store:etc/config.xsd">
    <default>
        <test_section>
            <test_group>
                <field_1>1st field default value</field_1>
                <field_2>2nd field default value</field_2>
                <field_3>3rd field default value</field_3>
            </test_group>
        </test_section>
    </default>
    <websites>
        <base>
            <test_section>
                <test_group>
                    <field_3>3rd field website scope default value</field_3>
                </test_group>
            </test_section>
        </base>
    </websites>
</config>