How Unicode Emojis Work
Every emoji is really a number. Unicode assigns each character a code point, and your device maps that number to a picture. Understanding this explains skin tones, flags and why some emojis appear split apart on older devices.
Code points
A code point is written like U+1F600 (the grinning face). Software reads the number and shows the matching glyph from an emoji font. If the font has no glyph for that number, you see a box.
Sequences
Some emojis are built from several code points joined together. A skin-tone emoji adds a modifier; a flag combines two regional-indicator letters; family and profession emojis use a zero width joiner (ZWJ) to fuse parts into one image. Where a device does not understand the sequence, it may show the separate pieces instead.
Why appearance varies
Unicode standardises the meaning, not the artwork. Apple, Google, Microsoft and others each design their own emoji set, so the same code point looks different depending on where it is shown.