Hi Niwantha,
When we are referring to regular plural nouns, we add suffixes such as -s, -es and -ies FOLLOWED BY an apostrophe s (s') to indicate possession.
Examples: Boy --> Boys --> Boys' treehouse // Fox --> Foxes --> Foxes' den // Baby --> Babies --> Babies' prams
On the contrary, when we are referring to irregular plural nouns, the spelling is changed from the singular to plural form. Hence, we SIMPLY ADD an apostrophe s ('s).
Examples: Man --> Men --> Men's issues // Child --> Children --> Children's toys // Person --> People --> People's ambitions
I hope this helps.
P.s. I am sorry for using Caps Lock for some words as I could not highlight anything in the "Answers" section.