Quality of hair depends on many things,mainly genetics but also diet and how well you look after your hair,how often you have it cut or wash it,whether you dye it or use heated styling tools on it.etc,etc.
There isn't much you can do to permanently thicken your hair but there are shampoos and styling products available to give the appearance of thicker hair.
or not.
i have very thick hair but my hair has thinned since menopause, and i've found that using thinning scissors on it just make it look even thinner, to the point you can see my scalp through my hair. my hairdresser now chops into it, rather than using the thinning scissors, to give it some texture.