Shopify themes and Liquid templating are designed with responsive design in mind.
When customers go to your store using a smartphone or tablet, its design should automatically change to provide the best browsing and shopping experience possible for the device’s screen size.
The Shopify Theme Store includes over 70 free and premium professionally-built themes that are designed to be responsive and optimised for a beautiful, intuitive shopping experience on all devices.
Shopify also provides a Mobile App SDK, which allows you to quickly and easily develop custom commerce apps—or to integrate Shopify’s checkout into your existing mobile application. With just a few lines of code, you can insert native Buy Buttons or embed a responsive online store into your app.
Designing responsive experiences on Magento 2 can be difficult given the complexity of page development. Magento includes a responsive theme for all merchants, but heavy page development and QA testing are required with each responsive or mobile deployment.