A lot of ordinary characters can be represented as two (or more) Unicode code points - for instance an unaccented Latin letter and a combining accent.

Flags emoji seem more like a hack on the side of the font or text renderer. If you look at the Unicode representation it actually spells out the ISO country code. Some fonts probably define a ligature containing these two characters that looks like a flag instead of two separate Latin characters.

Representation of digits inside keycaps also makes sense to me: it's a normal digit eight (dating back to ASCII) plus a combining character that looks like a keycap.