Google is now using what they call mobile-first indexing for many stores.
To understand this, you need to understand indexing first.
Indexing is just the process where Google has software that downloads copies of your store, analyzes them, and adds the analysis to their database. It's also called crawling because the program they use is called a website crawler or spider, you know... one that crawls the World Wide Web (who said geeks can't have a sense of humor).
In the past that program would pretend to be a regular browser and your host (Shopify) would just send a regular web page to it. That data goes into the database which gives Google the data to power the search results.
The mobile version of the search results would use the same database, just with a different layout so they are easier to read.
Mobile-first indexing flips this around.
Instead of the crawler asking for the desktop version of a webpage, it now asks for a mobile version of one. What it finds then gets added to the Google database.
For sites that created special mobile versions of their site with stripped down content and links, this can kill their SEO. That's because instead of seeing the full article with images, video, and everything else, the mobile version gets a very reduced experience. Often times with really poor navigation options.
Luckily with Shopify you're probably safe from this. The majority of Shopify themes are what's called mobile responsive which means it's the same website with the same content, just reorganized and sized for a mobile browser.
But in Shopify's past they used to support themes that are used for mobile-only browsers. They've been getting phased out because responsive themes are more popular and easier to use, but I've ran across one or two in the past couple of years.
If you are using a mobile theme with Shopify, you need to take the time to triple check that theme to make sure it has all of the content and SEO-ness that your main theme does.
The other part of mobile-first indexing that can impact all Shopify themes is the performance of the theme. Shopify themes and apps are notorious for poor performance. Google's been constantly harping on how site performance matters and mobile-first indexing is going to add more teeth to this.
Not only will a slow store annoy customers but it will impact your SEO performance too.
I'm not going to get into site performance much more here because that's a few hundred pages more I can write on it. Basically, you might want to start watching how your store performs just as you watch your conversion rates and traffic. Google Analytics has some reports on this (Behavior > Site Speed) and there are numerous tools to measure it too.
I've spent a lot of time making sure JSON-LD for SEO would work on mobile and also performs well, which meant it was ready for this change by Google.
But if you're not using it or your theme doesn't have structured data included in it, you could be at a loss. Especially since the mobile versions of Google rely on structured data and show Rich Results more often.
It's been used and tested on thousands of stores now since it first launched in 2015.
Eric Davis