Why the same emoji looks different on every phone
An emoji is not sent as a picture. It is sent as a number, and the receiving device draws the picture.
So the smiling face you sent and the smiling face they saw are two different drawings. Nothing broke. That is the design.
The number is shared, the drawing is not
An international standard called Unicode assigns each emoji a number and a name. It decides "the smiling face is number so-and-so" and stops there. What that number looks like is decided separately by Apple, Google, Samsung, and Microsoft, each drawing their own set.
Send an emoji from an iPhone and open it on a Galaxy, and you see Samsung's drawing. It is exactly the same thing as one sentence showing up in different fonts on different phones.
Usually the drawings are close enough that nobody notices. Occasionally they split badly. The famous case is the pistol emoji: in 2016 Apple changed its drawing from a realistic handgun to a toy water gun, and the other companies followed over the next few years. In between, a water pistol sent from one phone arrived as a real gun on another. Same number, completely different temperature of meaning.
An empty box means the device doesn't know that number
If an emoji shows up as a hollow box or a question mark, the receiving device has never heard of that number.
New emoji are added every year, and the drawings for them arrive through operating system updates. A phone that stopped getting updates receives the new number with nothing to draw for it, so it shows a box. On the sender's screen everything looks fine, so the sender never knows.
The only fix is an OS update on the receiving side. If the device no longer gets updates, that emoji will be a box there forever.
Some emoji fall apart into several
Many of the family and profession emoji are not one emoji but several, stitched together with an invisible joiner character. A family emoji is literally the emoji for two adults and a child, joined.
A device that doesn't know a particular combination cannot stitch it, so it shows the parts separately. You send one family; three faces arrive standing in a row.
Stickers are a different thing
Stickers and GIFs are image files. The file itself is what gets sent, so they look identical on every device. The looks-different problem belongs only to emoji, because only emoji travel as characters.
Where this bites
Since you cannot know how it renders on the other person's screen, any message where the meaning rides on the emoji is risky.
- An announcement or work document where one emoji is the difference between confirmed and
tentative
- Conveying a feeling through a subtle facial expression: each company draws the degree of the
smile differently
If the words carry the full meaning and the emoji is garnish, it reads fine on any device.
What this does not cover
- Skin tone and gender variants: they work by attaching a modifier character to a base emoji,
which is another combination, with the same falling-apart problem
- Emoji being hard to find while typing: that is an input-tool story
Searching the emoji board works by name: type "smile" or "check", click the one you want, and it's on your clipboard. The last eight you used stay at the top of the grid. Only base forms are collected, no variants, which are also the ones least likely to drift between devices.