One of the best ways to improve your Shopify SEO is to make sure you have enough valuable content for each product.
Ideally you'd have something unique that helps customers decide to buy, but sometimes you'll want to add some standard content too.
That's what Ambre asked me about the other day:
is there a way for me to add a specific sentence of text on each product description without going into every product page one by one?
There are a few options.
1. One way to add the content is to edit your Shopify theme for the product template. That's used for every product.
It's usually in the templates/ or sections/ directories though it depends on your specific theme.
This will not show up in the structured data though. If wanted to have it show up in the structured data too, you'd need to edit the actual product description. Google doesn't really use the product description structured data so there's not much benefit to that at this time.
2. You might be able to do it by exporting your products, editing their descriptions in a spreadsheet, and then re-importing your products.
But importing products in Shopify can be tricky. I've seen a lot of problems with it so I don't recommend this option most of the time.
3. You can also get a custom app built. They can pragmatically access the products and do a bulk update of the actual product description.
There might also be some inexpensive App Store apps that can bulk-edit your products but I haven't ran across any yet.
Of the three options, editing the template is going to be the easiest. Especially if the content is the same for every product.