C:\Program Files\dotnet\sdk\NuGetFallbackFolder

NuGetFallbackFolder stores a cache of NuGet packages used by an SDK during the restore operation, such as when running dotnet restore or dotnet build.

This folder is only used prior to .NET Core 3.0.

It can't be built from source because it contains prebuilt binary assets from nuget.org.

Apparently, this folder is supposed to be empty (most of the time?)