Named emojis are derived from their Unicode names; see /public/images/emoji/emoji_one for a list of all available emojis.
The default ASCII shortcuts are defined here:
https://github.com/discourse/discourse/blob/master/app/assets/javascripts/discourse/lib/emoji/emoji.js.erb#L101-L125
That’s also the file you would need to patch locally if you wish to customize your shortcuts… at least for now.