There Isn’t a Separate Index for Mobile and Desktop Indexing
Google have one main index where either the mobile or desktop version of a site is contained, this is the version which will then be shown in search results. However, if you have a seperate mobile site, Google will always show this version to users on a mobile device.
New Sites Will be on Mobile-first Indexing by Default
Google is now switching to mobile-first indexing by default for new websites.
Ensure Google is Able to Crawl All Resources Required for Mobile Friendliness
Mobile-first Indexing Rolled Out to Over 50% of Websites
Google has switched over to mobile-first indexing for over 50% of websites and the team is on track with regards to rolling this out to the remainder of websites.
Sites Are Only Moved to Mobile First Indexing When They Are Classified as Ready
Google has a classifier to measure if a site is ready to be moved to mobile indexing. If a site has not yet been moved to mobile first, it may be because the classifier has determined the site is not ready, or just that Google has not yet reached that site yet. Google is going to add some information in Search Console next year to highlight any issues preventing a site being moved to Mobile First Indexing.
HTTP/HTTPS & www/non-www Versions to be Switched to Mobile-first Index Separately
Mobile-first indexing is done on a per site level, meaning HTTP/HTTPS and www/non-www are treated separately and will receive notifications about being switched over individually.
Google Committed To Switching To Mobile-first On Step-by-Step Basis
Mobile-first Index isn’t running behind schedule. Google announced this was the direction they are headed in to give webmasters time to start making changes for the switch. Google is committed to switching over to the Mobile-first Index on a step-by-step basis for groups of sites that are ready.
Google Will Likely Move to Mobile-First Indexing in Batches
Google will likely provide provide guidance on when more changes can be expected. It’s probable they’ll move over batches of sites that work fine on mobile-first indexing first.