Google has made an official announcement regarding roll-out of mobile-first indexing. This is the first acknowledgment they have made through their own channels. Google’s representatives had earlier stated that many websites had already migrated to mobile-first indexing.
Google has once again stated that it is only migrating those individual sites which follow the best practices prescribed for mobile-first indexing. Sites which use dynamic or responsive web design are most suitable for mobile-first indexing. When websites have AMP as well as non-AMP versions of content, Google will give precedence to mobile versions of non-AMP page.