once和one 的用法?

one as a noun:- is the symbol; or figure
(1) a row of ones, ie:- 1-1-1-1-1.
--as in Book One, Chapter One, the first book, the first chapter.
--as in one day in the pop-song lyrical; one morning/afternoon/evening/night
--one summer evening=on a summer evening=on+indefinite article.
--one morning in June=on a June morning.
(2) The plural =ones---ones is used to indicate a contrast expressed, implied with the other, or another, or others
---eg:-The two girls are so alike that it is difficult for me to tell (the) one from the other.They are the ones.
---eg:-I do not know which to admire more, the one's courage or the other's determination. We use ones after "which" for the questioning statement as noun clause.eg:-Well, that's one way of doing it,know the ones.(=which are the ones)They are the ones.

把以下例句看完，你就懂了
We use one (singular) and ones (plural) to avoid unnecessary repetition.
正確使用one 或ones 避免相同字不重複使用
See those two girls? Helen is the tall one(=girl) and Jane is the short one(=girl).
Which is your car, the red one(=car) or the blue one(=car)?
My trousers are torn. I need some new ones(=trousers).
See those two girls? Helen is the one(=girl) on the left.
Let’s look at the photographs. The ones(=photographs) you took in Paris.

We often use them (one or ones) after Which ... in questions:
在疑問句，one 或ones 常接在which 後被使用
You can borrow a book. Which one (=book) do you want?
There are lots of books here. Which ones(=books) are yours?

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I know “ones” is the plural of "one", and I have searched the dictionary, but I do not understand the examples in the dictionary.
I hope you can give me some comparisons between "one" and "ones".