There are two options for using TastyIgniter with multiple restaurants, the Multi-Vendor extension (https://tastyigniter.com/marketplace/item/igniterlabs-multivendor) or Custom development of Multi-Tenancy.
You can purchase the Multi-Vendor extension from the marketplace, which allows you to turn TastyIgniter into something akin to JustEats or UberEats, where each restaurant has its own menu items with shared customers. However, there are limitations, such as each restaurant not having its own domain (retaurantA.example.com), but instead having (example.com/restaurantA).
The other option is a full SaaS, in which each restaurant has its own domain. A single TastyIgniter instance running across many domains. Where each restaurant has its own menu items and customers, for example, a customer registered with restaurant A must create a new account to order from restaurant B.
