You need to allocate licenses to the new users from A.

In Azure AD, license automation is implemented through group-based licensing. The SC-300 materials explain that “you can assign licenses to a group in Azure Active Directory; when you assign a license to a group, all users who are members of the group are assigned that license”. They also clarify that “dynamic group membership uses rules to automatically add or remove users based on user attributes”, which is the recommended way to on-board new populations without manual effort. For licensing, the guide is explicit that “group-based licensing works with security groups and Microsoft 365 Groups” and that “distribution lists are not supported for license assignment”. Administrative Units are described as “a scoping mechanism for delegating administrative permissions to subsets of users and devices”, not a licensing entity. Likewise, Organizational Units (OUs) are on-premises AD containers and “do not control license assignment in Azure AD”.

Given the requirement to allocate licenses automatically to new A. Datum users as they appear, the least-effort, policy-driven approach is to create an Azure AD Dynamic User security group with a membership rule that targets those users (for example, by domain, company attribute, or a synced attribute), then assign the required Microsoft 365/Azure AD licenses to that group. As the documentation notes, “when users join or leave the group based on the rule, licenses are added or removed automatically.”