By default .NET framework uses
packages.config. You can change this default in VS2017 or later by going to Tools > Options > NuGet Package Manager > General > Default package management format.
This default is used only when adding the first nuget package to a project. For existing projects that already have
packages.config, right-click the
packages.config file and click Migrate packages.config to PackageReference. This one-off migration will convert the project to use the newer format.