Hi,

You can add the button in the top bar of pos window by using following code.

<?xml version="1.0" encoding="UTF-8"?>
<templates id="template" xml:space="preserve">
    <t t-extend="OrderSelectorWidget">
        <t t-jquery="div.order-selector" t-operation="append">
            <div>
                <span class="order-button square add-product">
                    <i class="fa fa-files-o"></i>
                    Add Product
                </span>
            </div>
        </t>
    </t>
</templates>

For popup window based on this you can write the JavaScript code in the onclick function of this button. For an example you can check the following odoo app Create Products From POS .​ 

JavaScript tutorials are rarely available, you can refer odoo official doc  Javascript Reference odoo 13 

Regards