Emoji in apps

Because WhatsApp draws its own. Most apps use whatever emoji font the operating system provides, but WhatsApp ships its own artwork and uses that instead. The characters are identical, so what you send is unchanged; only the picture differs, and the recipient sees whatever their app draws.

In Slack, open the emoji picker and choose Add Emoji, or go to workspace settings and pick Customise. In Discord, open Server Settings and then Emoji, where you upload an image and give it a name. In both cases the result is an image scoped to that one workspace or server. Copy it anywhere else and you get the colon-wrapped name, because there is no character behind it.

They are a shortcode. Typing :fire: in Slack or Discord inserts 🔥, which is a typing shortcut rather than anything stored. The names are not standardised: Slack calls the hundred points emoji :100: and the red heart :heart:, so a shortcode that works in one app can fail in another.

Samsung maintains its own emoji designs and ships them in One UI instead of Google's Noto Color Emoji. Both are Android, so the same message can look noticeably different on a Galaxy than on a Pixel. Neither is wrong; Unicode standardises the character and leaves the drawing entirely to the vendor.

Next to a friend's name, yes. Snapchat assigns its own meanings to a small set of emoji to describe your streak and how often you message: a flame for a running streak, hearts for how long you have been each other's closest friend, and so on. That system belongs to Snapchat alone and has nothing to do with what the emoji mean anywhere else.

Not as emoji. Apple Color Emoji is licensed for Apple devices, so other platforms cannot ship it. Some apps let you send Apple-style images, but those are pictures rather than characters, so they will not be searchable or copyable as text.