Where we should create Azure AD B2C tenant in an Azure Landing Zone Architecture?

Azure AD B2C is Business 2 Consumer, so I am guessing you essentially will have a public facing website for customers to login to? If that's the case, you should make a DMZ subscription with internet traffic coming in via an app-gateway + waf. That's my opinion! The dmz vnet will be peered with the hub/connectivity vnet for management etc.

If I am correct in thinking it's for customers, then you setup the Azure AD B2C tenant & create an app registration and user flows for auth. This parts not really my strong point but this should help as a react example:
https://learn.microsoft.com/en-us/azure/active-directory-b2c/configure-authentication-sample-react-spa-app#31-configure-the-react-sample

It doesn't really matter where you create the Azure AD B2C tenant though... Think of it exactly like Entra ID.