Kate doesn't have naturally thick hair, more average-ish. It is wavy but not as much as you'd think. She had a curly blow dry during that South Seas tour and then it went frizzy, but people thought her hair was naturally that wavy.
When her hair is straightened, or doesn't have a fresh blow out, it isn't that thick. So there must be nets, hair donuts, sponges, hair pieces and extensions on occasion (the same for practically as famous women).
Her hairdresser was fired when he said that she had extensions. Her hair was thinner when she was dating William and then thicker when she was engaged and then became thicker and thicker over the years, sometimes thicker one week to the next. She also has the very best people working on her hair.
Help or not, she can be "absolute hair goals", as the kids say these days.