"Who's" is short for "who is", whereas 'whose" is used to indicated ownership.
So, for example,
"Mary is the contestant who's sure to win".
[Try putting "who is" in that sentence and you'll see that it fits]
. . . BUT
"Whose turn is is to pay for lunch?"
[Try putting "who is" in that sentence and you'll see that it DOESN'T fit].
So, if in doubt, the test is to try substituting the term that you were thinking of using with "who is".
If "who is" fits, you can use "who's" to replace it.
If "who is" doesn't fit, it must be "whose" that you require in your sentence.
Each of the large number of things growing on your head is a 'hair' (singular). In total, most people have about 100,000 'hairs' (plural) up there. However the entire mass is simple known as that person's 'hair' (singular).
Sometimes there can be a bit of an overlap in the terminology. For example, see this picture:
https://images.news18.com/ibnlive/uploads/2021/07/1627624394_hair-loss-1600x1200.jpg
The woman could be saying "Look at all the HAIRS on this brush" OR "Look at all the HAIR on this brush". BOTH are correct. (In the first case, she's referring to lots of single hairs. In the second, she's referring to the mass as a whole). However she'd only refer to what's on top of her head as her 'hair' (not 'hairs'), meaning all of it, if she went to a beauty salon and said "I think that my hair needs cutting".