For a little bit more information on how emoji requests are handled and the current requests being considered you can view the Unicode Consortium page here:
https://unicode.org/emoji/proposals.html
The reason why it is a third party that approves emojis is so they are the same across all device manufacturers. Each manufacturer simply adopts that latest standard emojis that have been approved. You can imagine the chaos that would be caused if each manufacturer created their own emojis, as you may send and emoji from one platform and it could be received as something completely different on the other.
Emojis are not actually pictures that are sent, they are simply a "character" just like letters on a keyboard and it is the manufacturer that interprets that "character" to be displayed on your screen as the emoji that has been standardized by the Unicode Consortium. That way the same "character" is represented as the same emoji no matter if you are using an iPhone or Android phone.