Those are standard emoji characters, which Cake displays using artwork we've licensed from EmojiOne.
In Windows 10 there's a keyboard shortcut you can use to bring up an emoji picker. In Windows 7 you may need to copy and paste them from a website like this one. macOS also has a nice built-in emoji picker you can bring up with a keyboard shortcut.
Eventually we'd like to implement a custom emoji picker for Cake so this will be easy no matter what OS you're using, but we haven't gotten to it yet. 🙂