Sometimes you'll happen across SEO advice that mentions modifying the
That file does quite a few things but it's also fully controlled by Shopify so you can't modify it. That's probably a good thing since it's a bit arcane and really easy to mess up.
Some things like hiding pages can be done outside of the
robots.txt by using the robots meta tag as described in this article.
It's more manual but it also ensures you don't remove your entire store from the search engines.
One thing that Shopify doesn't do is structured data.
Some themes have a bit but they usually don't quality for Google's Rich Results so your store loses out on highly visible SEO enhancements.
JSON-LD for SEO provides high-quality data that will let your store qualify for Rich Results. Often times faster than the industry averages.