You can add this to a <PropertyGroup> inside your csproj file to enforce copying NuGet assemblies to the build output:
<CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies>
However, note that the build output (bin/Release/netcoreapp*/*) is not supposed to be portable and distributable, the output of dotnet publish is. But in your case, copying the assemblies to the build output is probably very useful for testing purposes. But note that you could also use the DependencyContext api to resolve the DLLs and their locations that are part of the application’s dependency graph instead of enumerating a local directory.