It's been a few years now that Google started working on mobile-first indexing - Google's crawling of the web using a smartphone Googlebot. From our analysis, most sites shown in search results are good to go for mobile-first indexing, and 70% of those shown in our search results have already shifted over. To simplify, we'll be switching to mobile-first indexing for all websites starting September 2020. In the meantime, we'll continue moving sites to mobile-first indexing when our systems recognize that they're ready.
When we switch a domain to mobile-first indexing, it will see an increase in Googlebot's crawling, while we update our index to your site's mobile version. Depending on the domain, this change can take some time. Afterwards, we'll still occasionally crawl with the traditional desktop Googlebot, but most crawling for Search will be done with our mobile smartphone user-agent. The exact user-agent name used will match the Chromium version used for rendering.
In Search Console, there are multiple ways to check for mobile-first indexing. The status is shown on the settings page, as well as in the URL Inspection Tool, when checking a specific URL with regards to its most recent crawling.
Our guidance on making all websites work well for mobile-first indexing continues to be relevant, for new and existing sites. In particular, we recommend making sure that the content shown is the same (including text, images, videos, links), and that meta data (titles and descriptions, robots meta tags) and all structured data is the same. It's good to double-check these when a website is launched or significantly redesigned. In the URL Testing Tools