Since a long press on any key doesn’t bring any of those up, my method involved going to the text replacement section of the system settings, and doing a replacement entry. I copy the glyph from wherever I find it on the Internet and assign a unique string (the “shortcut”) to have iOS insert it. I’ve used a reliable pattern, such as (degc) (yes, including the brackets) for ℃. You need to choose a string that you will never otherwise use, otherwise you’ll be fighting against the text replacement.

Using this method I’ve added all sorts of special characters like fractions ¼ ⅙ ⅛ mathematical symbols ± « ≈ ≠ and even text emoji ಠ_ಠ ¯\_(ツ)_/¯ and other random symbols № ® ™.

Fun fact: if you have an AppleID-linked Mac, this will all sync over, letting you use these shortcuts on the Mac as well.