Global or Project specific libraries are independent of where they are located in your drive system.
Libraries can be stored anywhere you like, thoung there is a default directory (I do not know which is this directory in MacOS).
Having the libraries stored into a hard drive is not enough to have them available. The libraries must be loaded into KiCad too.
The attribute Global or Project specific is chosen by the user. It is the user who decides if a library is one type or the other.
Global means available by all KiCad projects.
Project specific means the library is available for that specific project.