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It's also worth noting that the sidebar is hidden in the mobile view.

To see the sidebar customer need to click on the minicart.

This can confuse the customer when he will not be able to quickly find the Place Order button.

Hiding is implemented in the following file with custom-slide class:

vendor/magento/module-checkout/view/frontend/web/template/sidebar.html

<div id="opc-sidebar"
     data-bind="afterRender:setModalElement, mageInit: {
    'Magento_Ui/js/modal/modal':{
        'type': 'custom',
        'modalClass': 'opc-sidebar opc-summary-wrapper',
        'wrapperClass': 'checkout-container',
        'parentModalClass': '_has-modal-custom',
        'responsive': true,
        'responsiveClass': 'custom-slide',
        'overlayClass': 'modal-custom-overlay',
        'buttons': []
    }}">

    <!-- ko foreach: getRegion('summary') -->
        <!-- ko template: getTemplate() --><!-- /ko -->
    <!--/ko-->

    <div class="opc-block-shipping-information">
        <!-- ko foreach: getRegion('shipping-information') -->
        <!-- ko template: getTemplate() --><!-- /ko -->
        <!--/ko-->
    </div>
</div>

I made VCT Place Order Sidebar (Button Lösung) on Magento Marketplace module that implements this task for mobile view as well.