On this page the wording is "Visual Studio subscriptions include one server license plus a user CAL for the subscriber". I simply cannot find additional information that explains this in more detail.
I understand that I need OS and DB licenses, I'm referring to DevOps Server specifically.
You can find Additional information here: Visual Studio Licensing Whitepaper May 2019. In Visual Studio Azure DevOps Server 2019 Licensing section:
You must acquire an operating system license for each machine running Azure DevOps Server 2019, any of the additional software, or the SQL Server database for Azure DevOps Server. You must acquire an operating system license even when your use of Azure DevOps Server is licensed as part of a Visual Studio subscription. For Windows Server-based deployments where Windows Server is licensed on a Server/CAL basis, each user or device that accesses Azure DevOps Server data (on a read or write basis) must also have a Windows Server CAL.
One instance of the Microsoft SQL Server 2019 Standard software can be used as the Azure DevOps Server 2019 database, except for Azure DevOps Server 2019 Express which uses SQL Server 2017 Express. Azure DevOps Server 2019 uses Microsoft SQL Server as its data repository and provides the right to deploy one instance of SQL Server 2019 Standard software per Azure DevOps Server 2019 server license. This is not a separate SQL Server license. This instance of SQL Server can run on a separate server but can only be used by Azure DevOps Server—not for any other purposes. If you do use the SQL Server software for purposes other than Azure DevOps Server, then you must license that use separately.
SQL Server Enterprise can be used for Azure DevOps Server 2019, but must be licensed separately. If you wish to use a different edition of SQL Server (such as Enterprise) as the Azure DevOps Server 2019 database, then you must acquire that license separately.
The part that Explains it is:
Visual Studio subscriptions. Visual Studio Enterprise Subscription, Visual Studio Professional Subscription, Visual Studio Test Professional Subscription, MSDN Platforms, and all Visual Studio cloud subscribers can download and deploy one instance of Azure DevOps Server 2019. These same Visual Studio subscribers are granted a Azure DevOps Server 2019 User CAL to be used within their organization (it is not valid for use of Azure DevOps Server acquired by a different organization).