We use one (singular) and ones (plural) to avoid unnecessary repetition.
See those two girls? Helen is the tall one and Jane is the short one.
Which is your car, the red one or the blue one?
My trousers are torn. I need some new ones.
See those two girls. Helen is the one on the left.
Let’s look at the photographs. The ones you took in Paris.
We often use them after Which ... in questions:
You can borrow a book. Which one do you want?
There are lots of books here. Which ones are yours?