What is mobile-first indexing?
Google continuously crawls the web to index and rank websites so that search results presented to users stay relevant. Google began indexing websites through the mobile-first Googlebot on July 1st 2019, meaning that any website that was new to the web or previously unknown to Google was indexed via the mobile version of the website first, and the desktop version second. Since the majority of users now access Google Search via a mobile device, it makes sense to crawl mobile first, so the search results will be more accurate to a mobile user. These changes will come into effect in September 2020.