As some one who works on a design system both professional and personally I would like to share some of my thoughts about design systems.
A design system is the single source of truth for you application. It includes guidelines for things like the icons, fonts, and colours that are used in your application.
It is a also a collection of reusable UI components that your developers can use to create the pages.
It can also include a file (for example a sketch file) that includes a library of resusable assets that can be used by UX designers to create their mockups and comps.
As mentioned above Design Systems are the single source of truth for everyone who has an hand in creating your application from the designers to developers.
This means everything in your application will be constent because everyone is using the same libarary of assets when creating there designs and common patterns will be defined for both designers and developers.
For the components when they are updated and the developers get a new version of the libary all the components though out the application will be updated automatically as long as there are no breaking changes.
Some of the tools that I use for creating design systems are as follows