I read on Microsoft Documentation https://docs.microsoft.com/it-it/nuget/consume-packages/managing-the-global-packages-and-cache-folders :
Whenever you install, update, or restore a package, NuGet manages packages and package information in several folders outside of your project structure
So, if I install Bootstrap Package for my solution MyFirstSolution the package should be downloaded on my folder global-packages and a reference to it will be added on my solution.
If everything is right when i create MySecondSolution and want to add Bootstrap VS should only add a reference to my project but technically I have to download it again from the package manager not only add a reference to the existing package downloaded for MyFirstSolution.
At the same time whenever I download a project by third part I have to reinstall all the packages used even if I already have installed it from other project made by me.