Google rolled out mobile-first indexing back in 2018 after testing it for more than a year, as more people search Google on a mobile device. Today they announced that they are making mobile-first indexing as the default for all websites starting September 2020.
Mobile-first indexing differs from traditional indexing in that, it uses the mobile version of a site’s content to rank pages from that site, instead of the site’s desktop version. When searching Google on a mobile device, there are possibilities that users will see a mobile page with less content when compared to the desktop variant due to how the traditional desktop Googlebot works.
Come September 2020, Google will switch completely to mobile-first indexing for all websites. Until then, when their system recognises a website is mobile-first index ready, it will automatically make the switch. With this decision, Google has noted that website owners will notice more crawling by both their mobile smartphone user-agent and traditional desktop Googlebot.
Most websites should be switched to the mobile-first index seamlessly. In the meantime, Google has recommended website owners to not use separate mobile URLs (often called “m-dot”) because it causes issues for both search engines and users.