I'm not sure what you're referring to with periods encapsulating the emojis.
As for the letters instead of country flags, it's because of Unicode codepoints combining, since country flag emojis are a combination of two emoji country letters, when they are written separate from each other, they show up as letters on their own. Here's a short article on that.

It also happens with some others, like skin color variants that are encoded by having a color emoji and then the standard yellow emoji (or the other way around, idk), or another that comes to mind is the heart on fire, which is heart emoji + fire emoji. These are joined with the Zero-width joiner character, you can try yourself to combine them, it's like an alchemical videogame!