How do you type the '¤' symbol on an iPhone keyboard?

For that you have to use an app like UnicodePad or just create a text replacement for it.

https://support.apple.com/guide/iphone/use-text-replacements-iph6d01d862/ios

Tom ~ I was expecting to find that '¤' symbol here, but no luck:

https://unichar.app/web

õ¿õ¬

That web page seems pretty incomplete. ¤ is called "currency sign", Unicode code point 00a4.

https://en.wikipedia.org/wiki/Currency_sign_(typography)

I can't recall recently seeing it used anywhere