A design system is the set of pieces the whole product is built from: colors, typefaces, spacing, buttons, form fields and cards. I define it as tokens, named values like “background color” or “corner radius”, that live the same way in Figma and in the code, as CSS variables or in the Tailwind config.
On top of the tokens come the components, each with its variants and states, and documentation explaining when to use each one. It's living documentation: it's updated along with the product instead of sitting in a PDF nobody opens.
Every project of mine is born with its own, however small. When a new screen is needed, it's almost done already: you combine pieces that exist and the brand stays the same everywhere. If your product already has a foundation, I can also tidy it up and document it instead of starting from scratch.