More common are multiple sites (which use their own domains for esthetic / brand reasons), but are actually hosted by the same SaaS provider and could therefore share authentication infrastructure.
Imagine an easy-to-use website builder for restaurants where each restaurant gets a memorable domain, and they let you order things online. It would be great for customers if they didn't have to enter their payment details and shipping address for each new restaurant they order from. Maybe they could even see opening hours and product availability for the closest restaurant to their address. There's no privacy risk here, as all these websites are actually on a single provider anyway. They're just multiple entries in some SQL database, each with a `domain` associated with them.
Imagine an easy-to-use website builder for restaurants where each restaurant gets a memorable domain, and they let you order things online. It would be great for customers if they didn't have to enter their payment details and shipping address for each new restaurant they order from. Maybe they could even see opening hours and product availability for the closest restaurant to their address. There's no privacy risk here, as all these websites are actually on a single provider anyway. They're just multiple entries in some SQL database, each with a `domain` associated with them.