Like many young people, my daughter is addicted to her phone, a Nexus 5 running Android 4.4. Absolutely no doubt, it is her single most prized possession. Mainly for Instagram, Texting and YouTube. When texting, she noticed that Android's emojis don't hold a candle to iOS. The iOS emojis are far more attractive. She wanted to get the iOS emojis on her phone.
So I did a little research. The emojis are a font stored in /system/fonts, called NotoColorEmoji.ttf. You can replace this font with any compatible font. However, the /system directory is mounted RO, so you need to root the device to mount it RW.
In a nutshell:
And even though I'm an old school command-line text kind of guy, I can't deny the iOS emojis are so much better than the Android ones. So I did the same with my own phone!
Now, where to get the Apple emojis? If you Google around you'll find it. Apparently the file is called something like Apple Color Emoji.ttf. Of course you'll rename it replacing the Android file.
As for rooting the Nexus 5 - this is a whole 'nuther topic. The official root method wipes the device. This is an understandable security precaution, since any app that became root to steal your data, wouldn't have any data to steal. But there's a way to root the Nexus 5 without wiping it, and without replacing or unlocking the boot loader. It's easy, simple and quick. It only works if you're running Android 4.4. There's a security hole in the Linux kernel that hackers exploited to secure root. There's an app called lambda that does this. Now you know enough to Google on this and do it yourself.