How to Extend Layout in Magento 2

Layout customization includes multiple tasks that help you change any elements in your pages. To customize your layout, create extending and overriding layout files in your custom theme.

Just add the next layout file in the layout folder like

<folder>

—Layout

–layout1.xml > layout2.xml

For each layout file that is included in the list:
Layout handle declaration and layout instructions are loaded.
The result is appended in the following format:

<layouts xmlns:xsi="XMLSchema-instance">
    <handle id="checkout_cart_index" label="Shopping Cart" type="page" parent="default">
        <!-- Layout instructions from checkout_cart_index.xml -->
    </handle>
    <handle id="checkout_onepage_index" label="One Page Checkout" type="page" parent="default">
        <!-- Layout instructions from checkout_onepage_index.xml -->
    </handle>
    <!-- ... -->
</layouts>

The corresponding layout file’s name defines a handle ID, and the the root layout node’s attributes of this layout file define handle attributes.Finally, replace the base URL placeholders in the result.