Such as the official Symfony Recipes server, Harmony use it's own flex server to serve private recipes for each extensions and packages available through the Harmony's Marketplace. This private server is used as a proxy/caching server of the official and contrib repositories.
This private recipes server can be accessed at the next url: https://flex.harmonycms.net.
Like Symfony, Harmony recipes are contributed by the community and are stored in a single public repository who contains all the recipes for the extensions and packages available through the Harmony's Marketplace.
This public repository can be accessed to the next url: https://github.com/harmonycms/recipes