Google officially prefers responsive web design when building smartphone-optimized websites.

"Sites that use responsive web design, i.e. sites that serve all devices on the same set of URLs, with each URL serving the same HTML to all devices and using just CSS to change how the page is rendered on the device. This is Google's recommended configuration."

Using responsive web design has multiple advantages:

It keeps your desktop and mobile content on a single URL, which is easier for your users to interact with, share, and link to and for Google's algorithms to assign the indexing properties to your content.

Google can discover your content more efficiently as we wouldn't need to crawl a page with the different Googlebot user agents to retrieve and index all the content.

Responsive websites adapt to fit smartphones, tablets, laptops, desktops and even smart tvs.