1. It's my phone.
2. It's mine.
3. The book is mine.
In sentence #1, we use the pronoun "my" together with the noun "phone".
In sentences #2 and #3, there is no noun because we don't want to repeat it.
You have to remember the forms of these possessive pronouns:
|Pronoun||Possessive determiner||Possessive pronoun||Examples|
||It's my phone.
||→ It's mine.
||It's your phone.
||→ It's yours.
||It's his phone.
||→ It's his.
||It's her phone.
||→ It's hers.
||It's our phone.
||→ It's ours.
||It's their phone.
||→ It's theirs.
Is it your hat? Yes, it's mine.
Is it our problem? No, it's theirs.
That's not my pencil. It's yours.
I show her my photos, and she shows me hers.
Let's learn questions with "whose".
Whose … is this? / Whose ... are these? = Who is the owner? Whom does this thing belong to?
Whose car is this? It's mine.
Whose tennis rackets are these? They're Kate's.
Whose desk is this? It's his.