For a while now, Google has been working on making mobile-first indexing the default behavior of its search engine. With mobile-first indexing, Google Search primarily uses a page's mobile content for creating its search index and ranking. Google announced this initiative in 2016 and as it announced today, by September 2020, it'll become the default behavior for all sites.
After a few small tests, the company started going all-in last year, and by December, it used mobile-first indexing for more than half of the web pages it showed in its search results. Today, that number is 70% already.
Google says most sites are now ready for its new system, but it will still occasionally crawl the desktop site with its traditional Googlebot. That means site owners may see increased crawling by Google in the coming months, too, as the company will now use two different crawlers: one that identifies itself with the mobile smartphone user-agent and one that will look like the Chromium version it uses to render the desktop site. For the most part, though, website owners will see the mobile agent.
For most sites, this switch should be seamless, but if you only expose structured data on your desktop site and not on mobile, it's now time to switch it on for both. Google also notes that it recommends that sites don't use separate mobile URLs "because of issues and confusion we've seen over the years, both from search engines and users."
Webmasters who want to make sure their sites are ready can head to Google's Search Console and check the status of their pages.