What's the difference between 'could' and 'could have'?

I was researching the uses of ‘could’ on this website, and I found these 4 sentences in the To talk about possibility or uncertainty section:

You could win, if you worked hard. (Possibility)

She could pass if she studied hard. (Possibility)

I could have helped him if he had asked me.

If I had the money I could buy a car.

Now, I have to ask that why they have written 3 sentences (1st, 2nd and 4th) using ‘could’ and one sentence (3rd) using ‘could have’, whereas all sentences have the same meaning (or, so, I think). What’s the difference between ‘could’ and ‘could have’? And, are there any similarities between ‘could’ and ‘could have’?

There’s one thing more. I’ve noticed, in 1st and 2nd sentence, they’ve used ‘could’ only (not ‘could have’) in the first part of the sentence and in the second part of the sentence, there’s no ‘had’. So, is it, like, this that when the other part of the sentence doesn’t have ‘had’, don’t use ‘could have’, just use ‘could’ and when the other part of the sentence has ‘had’, use ‘could have’ just as they’ve done in 3rd sentence.

Also, if that’s the case, then, notice, in the 4th sentence, ‘had’ HAS come, in the other part, but there’s only ‘could’ (not ‘could have’). I’m so much confused with these modals! Help me!