Some folks know the shortcuts for these things and seem to be able to magic them up at will. I don't, (and don't use software which auto inserts) so I just Google whatever weird character or symbol I need, copy it, and paste it (paste as plain text) straight into whatever document I'm writing.

These are the ones I use the most, which you can just copy from here any time:

½ ⅓ ⅔ ¼ ¾ ⅕ ⅙ ⅐ ⅛ ⅑ ⅒ ÷ × €

Generally, it's a bit quicker if you have a page bookmarked for the more commonly required rare (did I really just type that oxymoronic rubbish?) symbols like the fractions, so here's one:

https://en.wikipedia.org/wiki/Number_Forms

Currency symbols:

https://en.wikipedia.org/wiki/Currency_Symbols_(Unicode_block)

Mathematical symbols:

https://en.wikipedia.org/wiki/Mathematical_operators_and_symbols_in_Unicode

And here's a less useful link, which is a sort of massive index of 'Unicode blocks' if you have no idea what to put in when you're trying to Google it: