<type name="Magento\Braintree\Observer\AddPaypalShortcuts">
    <arguments>
        <argument name="buttonBlocks" xsi:type="array">
            <item name="mini_cart" xsi:type="string">
                Magento\Braintree\Block\Paypal\Button
            </item>
            <item name="shopping_cart" xsi:type="string">
                Magento\Braintree\Block\Paypal\ButtonShoppingCartVirtual
            </item>
        </argument>
    </arguments>
</type>

This is a type configuration.

This configuration is for the frontend area. This is the area that corresponds with the cart sub-application.

This configuration tells the Object Manager that when a user wants a Magento\Braintree\Observer\AddPaypalShortcuts object that the following arguments should be used in the foo object's constructor.

• $buttonBlocks = [...items...];