The Robots sections allows you to configure the robots.txt for each website. It will also add automatically the Sitemaps when defined in "Marketing > Sitemap"
To make the /robots.txt available to the public you have to add a new rule to the URL Rewrites for each store.
You can define a robots.txt in the "URL Rewrites" section.
- Crete a URL Rewrite from type "Custom"
- as Request Path use "robots.txt"
- as Target Path use "robots/index/index"
- Do not use Redirect and save.