Hello, I have always been thankful to you for great help and while I was reading your replies, a question about usage of parenthesis, ( ) came up

. Can you shed light on it?
the second is whether "90s" needs an apostrophe to form the plural ("90's"). like
The only one who didn't attend was Mr. Jensen (her father) So do you use the parenthesis for apposition with
the plural or do you just put it at the end for an additional information and then can we put each of the three at the end of each sentence, can't we?
Could you tell me the reason there is the parenthesis at the end? And while trying to solve this, I realized that there are many different usages about it between my language and English, so I am confused now. Please help me out again.
Thank you so much as usual!