You can't alter the B2C endpoint URIs but you can add the domain_hint query parameter to reroute the users to a specified domain. There are two samples that would be helpful for your use case:
This sample demonstrates how to implement a sign in journey, where the user is automatically directed to their federated identity provider based off of their email domain. And for users who arrive with an unknown domain, they are redirected to a default identity provider. https://github.com/azure-ad-b2c/samples/blob/master/policies/default-home-realm-discovery
This sample demonstrates how to create a home realm discovery page. On the sign-in page, the user provides their sign-in email address and clicks continue. B2C checks the domain portion of the sign-in email address. If the domain name is contoso.com the user is redirected to Contoso.com Entra ID to complete the sign-in. Otherwise the user continues the sign-in with username and password. In both cases (B2C local account and Entra ID account), the user does not need to retype the user name. https://github.com/azure-ad-b2c/samples/blob/master/policies/home-realm-discovery-modern
Let me know if these examples would help with your scenario.