Emoji are only supported in rich text formatted documents they're not actual text. If I type a Euro symbol it's a Euro symbol it's not a picture of a Euro symbol depending on context it's the actual Euro symbol. If I ask a computer what symbol it is the computer can tell me it's a Euro symbol, it doesn't go, ooh I don't know it's a picture.

€ 💶

One renders consistently irrespective of device viewing the other is entirely dependent of device viewing. Go look at this post on different devices and see the problem