As shown on this wikipedia page which I found for the #hashtag thread.
How long have any or all of them been in use and who decided they were necessary? Obviously this question is from a hopelessly innumerate and ignorant soul.
Various people at various times…
Happily enough, WP also has articles on the individual signs, so you can look up that ...
The equals sign (=) was invented by Robert Recorde in 1557.
Johannes Widmann gets early use credit for the plus and minus signs (+ -) (1498). Supposedly the + sign is a simplification of “et”.
The three dots meaning therefore (∴) date to Johann Rahn in 1659.
William Jones first used pi (π) in 1706.
and so on.