You cannot use Office Web Apps server with SharePoint Server 2019. You instead need Office Online Server. You can get OOS from the Volume Licensing Service Center under the Microsoft Office client products. Your users must be licensed with an Office Pro Plus with Software Assurance or M365 subscription that provides Office client. OOS can be installed on Windows Server 2012 R2 and 2016 with Desktop Experience -- Win Svr 2019 and Windows Core are not supported. You must install OOS on its own server not shared with any other application, such as SharePoint.
Visio is not supported by OOS; that is a SharePoint Enterprise feature. You need to create a Visio Services service application and start the Visio Service Instance on the appropriate servers (front ends).
As always, contact your Microsoft TAM, VAR, or licensing specialist for any licensing questions. You can search the PUR for specific licensing information, though again you should reach out to Microsoft or a reseller.
https://www.microsoft.com/licensing/terms/welcome/welcomepage
https://learn.microsoft.com/en-us/officeonlineserver/office-online-server