Possessive apostrophes are apostrophes (’) used with the letters at the end of a noun to show ownership over or a close connection with another noun. For example, if you were talking about the tail of your cat, you can add a possessive apostrophe and an s to show which noun is the owner.
its*
(It’s is it is)
“Its knees work hard because it’s severely overweight.”
It’s*
noun, possessive
The cat has knees. It’s knees are powder.
From your own source:
“When should you not use a possessive apostrophe? Do not use possessive apostrophes with pronouns, which have their own unique possessive forms.”
You wouldn’t use he’s or she’s or they’s for possession. It goes: his, hers, theirs, its. The cat’s knees = its knees.