I want to create a multi-tenant application where users will log in using Azure AD B2C. I will grant access to certain tenants that are our customers using policies. Only

It gets registered as a Enterprise application in other tenant So in there we can select Assign Users and Groups to give access to this Application from their tenant or they can set conditional access policy as well for specified set of conditions.

Reference:

Restrict Azure AD app to a set of users - Microsoft identity platform | Microsoft Docs