My name is Ardiane, a Guru here at Shopify!
The Shopify Buy Button is essentially a way of adding eCommerce to a website by embedding a Buy Button that's connected with Shopify checkout. It's built to be an addition to a site and can't be created into a separate site itself. If you'd like a separate site, you would need to build an online store and not just have the Buy Button. Here's a doc that explains what the Buy Button is in order to help you understand what it's used for.
- Ardiane :)