If your organization has a Visual Studio subscription (any tier), you can access original product ISOs, including all versions of Windows Server 2016 (RTM and updated builds).

Never download "pre-activated" ISOs – they are illegal and dangerous.

When you boot from the ISO, you will typically be prompted to choose between different editions. Selecting the correct one is vital, as converting between them later can be difficult. Comparison of Windows Server editions | Microsoft Learn