What is a ZWJ emoji sequence?

A Zero-Width Joiner (ZWJ, U+200D) sequence combines two or more emojis into a single composite emoji. For example, 👨 + ZWJ + 🚀 = 👨‍🚀 (man astronaut). Family emojis (👨‍👩‍👧‍👦), profession emojis, gender variants and skin-tone combinations all use ZWJ.