Emoji get approved through the Unicode process. A proposal usually explains what the emoji represents, why people would use it, how often the idea appears in real communication and whether it is visually distinct.
Unicode looks for emoji that can be used broadly, not just one brand, meme or short-lived trend. The proposed emoji also needs a clear name and a design idea that can work at small sizes.
After approval, the emoji becomes part of a Unicode release or emoji data release. Platforms then decide when to support it in fonts, keyboards and apps.
Approval does not guarantee every device will show the emoji immediately. Until support arrives, a newer emoji can appear as a missing box, separate characters or a fallback symbol. This connects to why emoji look different.