Google has announced by this week to switch mobile-first indexing and crawl for all websites starting September 2020.
Google has announced to roll out their experiments to make their index mobile-first back in November 2016. In the context of mobile-first indexing, Google will evaluate the actual page content that is seen by a mobile searcher for their ranking systems. And this will make their search result more useful for searches using a mobile device.
Ever since then, Google moving sites which are ready to mobile-first indexing, using smartphone Googlebot for more than 3 years. Today, Google gained enough confidence that the effort had bought great user experience, and decided to simplify by switching to mobile-first indexing for all websites from September 2020. According from Google, 70% of websites on their search results have already moved over this indexing practise.
You can check it from Google Search Console. Once you are in the Search Console, select the property and then check on the Setting page. You will be on the view as illustration below: -
From the illustration above, you will see the red color highlighted section showing that the property has already switched to mobile-first indexing. Meanwhile, Google will continue to move your website(s) to mobile-first indexing when their systems recognize your website(s) are ready.
While Google is updating their index for your site's mobile version, you will see an increase in Googlebot's crawling. Google's mobile smartphone user-agent will take place in most crawling for Search after this, but traditional desktop Googlebot will still do crawling occasionally. Website owners will then see mobile Googlebot's crawling rate more frequent over desktop Googlebot.
Google recommended to have same content for both desktop and mobile version of your website, which including text, images, links, videos, meta data such as title, description, and all structured data of each corresponded page.
Despite of Google will continue to support different ways to make and format of mobile website, Google highly recommended to go for responsive web design website. Google strongly suggest not to use separate URLs for both desktop and mobile version of your website (e.g.: m.mydomainname.com as your mobile website URL, and www.mydomainname.com as your desktop website URL), "because of issues and confusion we've seen over the years, both from search engines and users."