Clarification on B2C Charging and Monthly Active Users

It sounds like a permissions issue when you're trying to access the cost UX in the portal. This has nothing to do with your billing for B2C. Most likely the account you're accessing the portal with doesn't have permissions to see the billing information for the subscription the B2C tenant is attached to. This is pretty common as generally only the folks paying the bill need access to it. However if your B2C tenant was set up before Nov 2019 then you might be on the older payment model and need to switch as discussed here.

If you do need to see the bill then you would need to talk to your Azure admin and ask that they grant you permissions to see the billing information. See here for the permissions that may be needed.

As for the "active users" you can read the details here. Basically an "active user" is any user who connects to B2C during a month. That connection would include creating an account, logging in, etc. Note that it applies per unique user (object-id). So if you have 1000 users create an account or log in during a month then the AU is 1000. But if a single user logs in 1000 times then it counts as 1. My gut instinct is that it based upon the object-id of the user which is generated at the point the user is created. Basically you are getting charged for each unique user who uses B2C each month rather than the older model which charged you for every user in B2C whether they actually used the services that month or not.