Apple does not provide any of the Emojis you see. Those are maintained by the Unicode Consortium where you can request new emojis there.
https://unicode.org/emoji/proposals.html
The reason why that is the case is so the emojis are compatible across all device platforms. If they were not maintained by a third party, then each manufacturer could have their own emojis where you could send one on your device and it could appear as something completely different on another. The device manufacturers simply adopt the latest approved emoji set from the Unicode Consortium.
Emojis are not actually pictures that are sent from one device to another, they are just a unicode, like any character on the keyboard, and when the device receives that code it knows how to display that character/emoji.
Newer Apple devices are capable of using a feature called Genmoji, which lets you create a picture of your choice and it is used similarly to an emoji. A Genmoji is actually a picture though, so it does not suffer from the same compatibility restrictions of using an Emoji.