For now GitHub clone is recommended approach. Hope they will move foundation to dotnet project templates soon.
More info - https://world.optimizely.com/blogs/Mark-Hall/Dates/2021/8/starting-a-new-project----net-5-series-part-2/
If you prefer to start with one of the foundation projects we have .Net 5 branches available. These branches will move to develop once the software is General Availabiility.
Although Alloy/Quicksilver was in the .NET 5 preview it's unlikely these will be there moving forward so foundation is the way to go at the moment.
It could be that Optimizely keeps Alloy also in the works at some level (for .NET x version(s)): netcore-preview/issues/125
I have installed and are running en empty CMS 12 project, but how do i install either Foundation or Alloy?
Is this article relevant? https://world.optimizely.com/documentation/developer-guides/CMS/getting-started/install-a-sample-site/
Should I just clone the GitHud Foundation, that does not sound correct for .NET Core?