Good explanation of how new emoji are approved and made available to modern devices by Jonah Bromwich at The New York Times:

In deciding which emojis to add, the Unicode Consortium considers factors including compatibility (if a pictorial symbol in broad use has not been translated into Unicode), and frequency of use (whether people will be interested in using a certain picture often enough to justify its existence).

Another factor is “completeness.” For instance, at one point, the group added a mosque, a synagogue and a generic place of worship to complement the Christian church symbol that was already included.

Side note: The New York Times pluralizes emoji as “emojis”, but I use “emoji” for both singular and plural variations. That seems to be Apple’s preference, too.