How to create certain two-character items with the Unicode Hex keyboard?
In my TextEdit in 10.13, I can get the French flag, which is 1f1eb 1f1f7, by typing the utf-16 for each of these separately with the option key held down, and they do combine:

D83CDDEB D83CDDF7  

(To find out character components of glyphs that display as a unit you can use the app UnicodeChecker)