If you use one of O365 SKU's, see ( ) for details on activating the VDA benefit.
Or, you can license VDA through some of the Office 365 SKU's like Microsoft Windows 10 Enterprise E3 VDA (which covers Win10 Enterprise upgrade rights and VDA usage rights) or Microsoft 365 E3/E5 (which covers Win10 Enterprise upgrade rights, VDA usage rights, the O365 services, and a couple of other bells and whistles).
If you purchased the Win10 Pro licenses through a retail channel (like having Windows bundled with a new computer), have a lapsed SA agreement, or your users are BYOD, you can purchase the VDA license separately from a reseller using the Open, Select, etc licensing programs. Spin up those Win10 VM's and pull the license keys from your Volume Licensing Center or add them to your KMS server or however you manage keys). If your client operating systems are covered by an active SA agreement, congrats! You're all done. Win10 licenses purchased through one of Microsoft's plans like Open License, Open Value, Select, etc include 3 years of SA (with the option to renew when it expires). Licensing a client OS (as opposed to server OS) virtual machine requires either an active Software Assurance (SA) or Virtual Desktop Access (VDA) license.
I do a fair amount of licensing work at an MSP/CSP so here's my understanding (your second link with the PDF from Microsoft has most of the information but I'll add this and this for additional reference).