During setup you can choose Standard or Datacenter
Windows server 2016 iso download link.
https://www.microsoft.com/en-us/evalcenter/evaluate-windows-server-2016/
Related to downgrade from win2019 to win2016, refer below link
https://techcommunity.microsoft.com/t5/windows-server-for-it-pro/windows-server-2019-to-2016-downgrade/m-p/1314278
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