If you exclude 'emojis' which are literally just picture forms of characters in existing languages:
Technically, they're pictoglyphs, not hieroglyphs.
Emojis represent the literal thing they portray (pictogram), the word associated with that literal thing (logogram), and, to varying degrees to different people, represent other related concepts and associated ideas (ideogram).
Hieroglyphs do all of that, but additionally also represent spoken syllables and alphabetic characters, and follow a concrete syntax amd grammar system that allows for a much more extensive vocabulary, as well as more complex sentence structures than emojis, which have far more certain and unambiguous meanings.
As an example of that, try to represent my previous sentence using only emojis, and then present that string of emojis to a random person you know and ask them what it means.
Basically, hieroglyphs constitute an actual language, purely using emojis is playing pictionary as a means of communication.
When I was in high school, my mate didn't even have a phone. We just knew that Friday at 7pm we met at the standard spot and went to grab a beer. We didn't even make plans before, it was just how it was.