https://support.google.com/analytics/answer/12799087
Actually that link is not so useful on its own, that one is more about setting up gtm on your website.
So into gtm you generally push data as key-value pairs. As for they keys, websites have freedom to use anything they like, and integrate with other companies who would ask for any keys THEY want, but you can probably make some pretty good guesses. As for the values, there is a standardised list/taxonomy of "audience segment id" codes provided by the IAB. They probably have one for cheese.