Trying to experiment with design tokens... how do you actually consume them?

It's easy to find resources talking about design tokens but they usually talk about the concept and how to generate your tokens, not how to actually set them up end-to-end. I'm fuzzy about a number of things:

* When do they come into play? At build time, at run time? * Is there a standardized format? At first glace Figma export looks like a plain JSON data file but I've seen some that had variables in there with handlebar notation which means reading them will require extra work. * Are there any good libraries that deal with this?

Has anyone successfully applied this idea and can share their experiences?