So you want to use System Core as a dependency for your assets? That's great!
Yes the MIT license with its common clause does allow you to create assets that are dependent on System Core. The common clause simply aims to prevent people from copying or deriving from System Core code to release a competitor to System Core paid or otherwise.
If you would like to make a better System Core to compete with us … again that is great, competition makes the world go round!
But you need to start from your own code not ours.
This article will help you get started, there isn't much to cover but what there is; is important to know!
System Core should only ever be installed via Package Manager! You can check rather System Core is installed and what version is installed using the Package Manager namespace in Unity.