What is the difference between 's and s' in plural possessive nouns?

see below

The 's is added to form the possessive of plural not ending in "s".

Ex:

children 's toys

men 's work

For plural nouns already ending in "s", there is no more need to add s . Instead, an apostrophe ( ' ) is placed after the s . Because of this, it appears like an s' is added.

Ex:

boys ' game

teachers ' party