Where do I find my sitemap, and can I edit robots.txt?

Both are in Settings → SEO Settings → SEO Tools.

• Sitemap: your store has a sitemap at /sitemap.xml on your domain, which links to separate sitemaps for products, categories, brands, pages, blog posts and tags. It updates itself as you add things. Use Copy and submit it in Google Search Console. Show individual sitemaps lists the separate ones.
• Robots.txt: generated for you (allow all, plus your sitemap). Click Edit to write your own; it is served exactly as typed. Leave it blank to go back to the default.
• Blog RSS Feed: a feed of your latest posts.

If Search Engine Visibility in General Settings is set to No, robots.txt blocks every crawler whatever you write there.