Google Recommends a Single Site for Mobile First Indexing
It’s OK to have a separate mobile website for mobile first indexing, provided it’s correctly linked from the desktop version with a mobile rel alternate and a canonical pointing to the desktop page, but Google recommends having a single URL for each piece of content with responsive design or dynamic serving in the future.
Canonical Tags Should Remain The Same Between Desktop & Mobile Sites Even With Mobile First Indexing
If you have a separate m. version of your site, the canonical should remain pointing to the desktop version, despite mobile-first indexing. This is because Google will use the canonical tag to understand which of the pages belong together. Internally, Google will pick the mobile version as canonical.
Google Advises Against Using Separate Mobile URLs
Having separate URLs for the mobile and desktop version of a site adds more complexity for website owners to manage, so Google recommends switching to a responsive setup.
Implement Redirects From Mobile Pages to Desktop Pages For Desktop Users
If you have a separate m-dot site, Google will usually pick this as the preferred canonical version after mobile-first indexing and the m-dot site will be shown in desktop search results. To avoid negative UX, implement redirects to the desktop version for desktop users.
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.
Mobile-friendly Errors on Desktop Page Could Mean Google Doesn’t Understand Connection With Mobile Version
If mobile-friendly errors are reported on for the desktop version of a page which has a rel alternate to a mobile version, this could mean that Google doesn’t have a clear understanding of which pages belong together and they could be indexed separately.
You Don’t Need to Submit Separate Mobile URLs in Sitemaps
URLs for separate mobile sites don’t need to be submitted in their own sitemap. If the rel alternate tags are set up correctly then Google will be able to find these pages through their connection to their desktop equivalents.
Poor Internal Linking on Mobile May Impact How Google Indexes and Ranks Pages
Poor internal linking on the mobile version of a site could impact the way Google index and rank the site after it has been switched over to mobile-first indexing. You can use tools like DeepCrawl to crawl your site with a mobile user-agent.
Move Disavow File to Your Separate Mobile Site for Mobile-first Indexing
In the same way that you should move the disavow file across after a domain or HTTPS migration, you should upload the disavow file on your separate mobile site if it was previously uploaded just on your desktop site.