Since July 1, 2019, Google predominantly has used the mobile version of the content for indexing and ranking.

“Mobile-first indexing is enabled by default for all new websites (new to the web or previously unknown to Google Search). For older or existing websites, we continue to monitor and evaluate pages based on the best practices detailed in this guide. We inform site owners in Search Console of the date when their site was switched to mobile-first indexing.” – Google

These are some of the best practices to make your website mobile-friendly, declared by Google itself:

• Use the same meta robots tags on the mobile and desktop site.
• Don’t lazy-load primary content upon user interaction.
• Let Google crawl your resources.
• Make sure that your mobile site contains the same content as your desktop site.
• Use the same clear and meaningful headings.
• Make sure that your mobile and desktop sites have the same structured data.
• Use correct URLs in structured data.

There are still other ways to make your website mobile-friendly. Google even also has provided some tools in their Google Search Console that can audit your website if it is already mobile-friendly.