How do you use these on iPhone? 
Discussion
Not sure. I think it is up to the client support. It took some time for Amethyst and snort shows them on web, as well as Rabbit(also web which supports making your own emojis). Not sure if Damus will or already does.
Oh ok
You can login into the vercel app from web with extention and post a message from there along with the emojis (the format is :nameofemoji: and will be in the note, not as a reaction, the name of the emojis are in the packs or you can make your own emojis). It will show on your damus nostr profile as well as a note. How will damus display it, well up to test ๐ค