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.