Microsoft Learn | Azure built-in roles - Azure RBAC:
Desktop Virtualization User: The definition states this role "Allows user to use the applications in an application group." This confirms the role must be applied to the application group resource.
Virtual Machine User Login: The definition states this role allows users to "sign in to an Azure virtual machine with regular user privileges." This is the second required permission, applied to the VM.
Source: https://learn.microsoft.com/en-us/azure/role-based-access-control/built-in-roles (Sections for Compute and Desktop Virtualization)
Microsoft Learn | Delegated access in Azure Virtual Desktop:
This document confirms that Azure Virtual Desktop uses Azure RBAC and that the Desktop Virtualization User role is assigned to a user or group with the scope of the application group.
Source: https://learn.microsoft.com/en-us/azure/virtual-desktop/delegated-access-virtual-desktop (See section "PowerShell cmdlets for role assignments")
Microsoft Learn | Sign in to a Windows virtual machine in Azure by using Microsoft Entra ID:
This document details the requirement of assigning either Virtual Machine Administrator Login or Virtual Machine User Login roles to users who need to sign in to the VM.
Source: https://learn.microsoft.com/en-us/entra/identity/devices/howto-vm-sign-in-azure-ad-windows (See section "Configure role assignments")