Universal Containers (UC) is in the process of implementing a B2C Commerce site for their...

This answer is correct because it is the primary system that should be used for all authentication between B2C Commerce site and Experience Cloud site. Salesforce Identity can act as an identity provider (IDP) for both B2C Commerce and Experience Cloud, and it can provide a single login and data experience between the two sites. Salesforce Identity can also store customer profile data in a single place and sync it across systems. References: https://help.salesforce.com/s/articleView?id=sf.b2c_commerce_service_cloud_integration.htm &type=5