How to do eternity-infinity sign?
If you don't want to rely on third-party solutions (keyboards) like the one mentioned by @TomGewecke, use the built-in functionality for Text Replacements, as mentioned in the article you already linked to.


First, copy the Unicode symbol, e.g. from here.
Go to the Settings app -> General -> Keyboards -> Text Replacement -> create a new one with the + button in the top right.
Paste the symbol in the Phrase field, and an appropriate term in the shortcut. I decided to go for the HTML entity &amp;infin; since that works in Markdown as well: &infin;, but you can definitely choose a shorter/easier one.


Save the text replacement, and now whenever you type &amp;infin; the keyboard auto-complete suggestions will offer you to replace it with &infin;: