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.
One of the hallmarks of a destabilizing and imminently pre-collapse ecosystem is when certain fast lived species like insects have sudden surges or collapses in population.
And I’m talking about short-lived species that typically have yearly cycles. Something that can respond very quickly to sudden surges or absences in food or environmental niches, but which does not normally see sudden population fluctuations in a healthy ecosystem.