[gumby]

The ideal referral code is a clickable link. It's the same reason domain names are fading: friction from issues like being too easy to mistype, hard to remember, scammers etc.

Most apps let you message or mail a referral link to a friend.

[dontmitch]

That doesn't work all that well on mobile apps, since users first have to download a new app before you can record the referral code. Having a link click persist through that process is nearly impossible.

[gumby]

Make the link go to a web page with a "download app" link -- the app can start and look for a cookie left by the web page.

Or simply download the app and click again.

I'm not saying not to have user-readable ref codes (though emojis are a terrible choice -- you can't read them aloud), I'm saying make typing them the very rare case.