Visual Studio subscribers are entitled to Visual Studio subscription features as a subscriber benefit. When you add those users, be sure to assign them the Visual Studio subscription access level.
Or does the billing system, by itself, figure out that the user has a
VS subscription, even though I force a basic license for that user.
The system automatically recognizes their subscription and enables any other features that are included, based on their subscription level. You don't have to pay for Users with recognized VS license: https://learn.microsoft.com/en-us/azure/devops/organizations/security/access-levels?view=azure-devops#visual-studio-subscription-access
DevOps Basic Access Level: First 5 users free, then $6 per user per month

Visual Studio Professional subscription has DevOps Basic benefit.
Visual Studio Enterprise has DevOps Basic + Test Plan benefits.
VS doc: https://learn.microsoft.com/en-us/visualstudio/subscriptions/vs-azure-devops#eligibility