How does GitHub make commits with flag icons?

Question

I saw an interesting project on github today - it has a commit that starts with the British flag, as shown below. When I move the mouse up, I can see :gb:. How do you do this? What about the Chinese flag?

Project address: https://github.com/wagerfield/flat-surface-shader

Answer 1

Addition to @joyqi’s answer.

I just learned that there is a detailed emoji list on the page below. Just use the corresponding characters to express it
http://www.emoji-cheat-sheet.com/

Answer 2

Just use the :cn:special mark

Answer 3

Testing works