Emoji are part of the Unicode system. In the actual message your phone sends, they’re represented by a number along with a prefix that says “this is Unicode.” When the receiving phone gets a number that starts with this prefix, it knows to go to a library and look up a symbol using that number. Then it displays that symbol. If the phone’s library is missing or incomplete (Unicode is biiig, and it would take a lot of space to store all those pictures), then it shows a blank character, or possibly just the Unicode number it got.
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