It really depends on what SEO features each app has. They all vary. Out of the box, Shopify does a lot of things well for SEO, but it's not everything. This isn't necessarily a flaw as they have to focus on the features most merchants benefit from. Looking at one app https://apps.shopify.com/seo-meta-manager, you'll see you can get a log of 404 errors and json-ld markup. Some themes may have schema markup already done that make json-ld unnecessarily. But these are some features not native to stores.
A lot of features in that particular app can be done yourself like GSC validation, mobile-friendly testing, sitemap submission through Google's tools, but obviously, not all merchants have such knowledge and like being able to do it through such an app.
In case you are looking for a SEO app for your Shopify store, have a look at this SEO Suite App from Avada Commerce. The app helps you to build on-page SEO effortlessly. It is designed to optimize website images, content, and structure so that Google Search Engine can find you easily and rank you better. And more important, this app is FREE FOREVER!
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Shopify comes with several SEO-friendly features right out the box, but there's so much more you can do to climb up in the rankings. Apps will make it easier for those who don’t have the time or expertise to do SEO.
One of the main step in SEO is to regularly submit your store feeds to Google. Before you think of ranking you should ensure your pages are found (indexed by Google). You can submit your store content regularly to all the major engines, or consider using a convenient tool like flare by Mansion Ecommerce. flare automatically detects site changes and verifiably informs major search engines like Google & Bing. You can get your free Google Index Score through flare. Plus! Only flare can submit individual URLs to Google & Bing, allowing your prioritized pages to be indexed first and fast.
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Shopify stores do come with SEO ready features, however to do a deep dive into SEO and achieve better rankings, more detail is needed within the realm of SEO.
SEO apps do streamline these things, however some level of manual work and research should also be done. If you're looking for some more basic tips on Shopify SEO, feel free to read this short article we've written to get you on the right track: https://syntechdigital.com/shopify-seo-tips/