With Unicode Hex Input, I can put glyphs with Unicode values of more than two bytes into my documents by entering four bytes with leading zeroes.
Flag emoji (and other things) are defined as the proper rendering of two adjacent five-nibble codes. When I type them into TextEdit, they do not combine. Other two-character items will render the first item when it is complete, but change it when the second is complete.
These particular items just show two unknown boxes when their codes are typed. But if I copy/paste the same flag from elsewhere, it is rendered correctly. When the source was a web page, "show source" gives an actual flag and not a numeric entity.