iOS: How can I view more Unicode? I would like to be able to view more Unicode characters on my iPhone—specifically, more mathematical operators and symbols. Most of these are not visible on iOS and either appear blank or as a question mark inside a box despite being directly accessible through the character viewer on a Mac.

How can I view these characters on iOS?
You would have to add fonts to your iOS device which contain the characters which are missing.  This can be done with various apps from the app store, like Anyfont.

You might try the font Symbola.