Unicode emoji are emoji defined by the Unicode Standard. Unicode gives computers a shared way to identify text characters, symbols and emoji across different systems.
When you send 😀 Grinning face or ❤️ Red heart, the message contains Unicode data. The receiving device uses that data to show its own emoji artwork.
Unicode also records sequences, including flags, skin tone combinations and ZWJ emoji. That is why some emoji are a single code point while others are built from several characters.
For the basic idea, start with what is an emoji. For combined emoji, see what is a ZWJ emoji sequence.