Fonts installed not showing up in General>Fonts

You can normally only use downloaded fonts on apps which have a font menu, you can't change the font for the phone's general user interface.

I have found downloaded fonts sometimes do not appear in General > Fonts, but that doesn't matter in terms of how they can be used, which is quite limited.

Your ability to type 𝖙𝖍𝖎𝖘 is the result of a sort of trick using special unicode characters intended to be used in advanced math equations and formulas (much like adding emoji or other symbols to your text).

You need to download the Fonts through an App purchased from the App Store.

I am not sure how you directly downloaded it from the internet. Is the device Jailbroken?

PS The special characters represented by 𝖙𝖍𝖎𝖘 are mainly used in Lie Algebra.