@EdRick - Those are really good questions! The possessive pronouns are indeed the words that can stand on their own, like "mine" and "hers."
Some words have the same form whether they are being used as a possessive pronoun or a possessive adjective, and those are "his" and "its." So you could say "My dog is bigger than his dog" or "My dog is bigger than his" and both would be correct.
"Whose" is an interrogative pronoun, an interrogative adjective, or a relative pronoun, depending on how it is used. So if you said, "Whose are these shoes?" you've used it as an interrogative pronoun, but if you said, "Whose shoes are these?" you've used it as an interrogative adjective. And if you said "I gave a ride to the girls whose mother couldn't pick them up," it's a relative pronoun.