Out of the box the dotnet pack command (from the .Net Core CLI) treat every project in the Visual Studio solution as its own Nuget package.

Project-to-project references aren’t packaged inside the project. Currently, you must have a package per project if you have project-to-project dependencies. - .NET Core CLI Docs

Today i learned that if you add PrivateAssets="all" to the ProjectReferece in the .csproj file, these project will be included in the Nuget package, instead of marked as a dependency.

This only works for .NET Core or .NET Standard build with the .NET Core CLI and not .Net Framework build with MSBuild.