Emojis are color glyphs which follow the Unicode standard.
They are present in three blocks:
Other symbols available in different blocks (like Maps and Transportation) can be treated as Emojis and be designed as color glyphs. You can find all the code charts here: https://www.unicode.org/charts/
To create a font with color Emojis you need an editor like FontLab 7. FontLab Studio 5 does not support color glyphs.
Documentation for the new FontLab is available, although it's under the development: