New emoji are standardized through Unicode. People and organizations can submit proposals, and Unicode reviews whether an emoji is likely to be useful, distinct and appropriate for encoding.
Unicode decides the character or sequence data, not the exact artwork. After an emoji is approved, companies such as Apple, Google, Microsoft and Samsung create their own visual designs.
This is why 🧑 Person, 👩 Woman and 👨 Man can share a general meaning across platforms while still looking different in each emoji font.
Approval is only one step. After Unicode adds an emoji, each platform still needs to ship support before users can see and type it easily. See how emoji get approved for the process.