It's exactly like shopify's setup (and pretty much all other ecom platforms).
You register your business and they automatically provision
mybusiness.shopify.com. You then start designing your shop etc. At some point it's ready and you want to sell, but look professional so you want your own domain (possibly several for different sales channels). So you point
mybusiness.com at
mybusiness.shopify.com and that's the site you send customers to. the subdomain merely acts as a quickstart.
So customer A goes to
mybusiness.com and see's some awesome product they want. They add to cart, and at checkout there's a "Have an account? Sign in here" button. This directs them to
mybusiness.com/signin. We want the same customer to have a seamless experience across stores and have a "powered by xxx" link which explains all websites using our backend have a secure SSO which saves them time and means they dont need to remember a log in for every independent store they shop at