How do flag emojis work?

Flag emojis are regional indicator symbol sequences. You take the two-letter ISO country code (US, FR, DE), replace each letter with its matching regional indicator (🇺, 🇸 for US), and concatenate: 🇺 + 🇸 = 🇺🇸. That's why your device can display flags it wasn't specifically designed to — the OS composes them from the two letters.