Stevia is a herb, but the sweetener called Stevia is not stevia-the-herb. The sweetener is one of two chemicals that are produced by the herb. These chemicals are extracted from the herb, purified, and sold as sweetener.
The sweeteners are called Steviol Glycoside, and Rebaudioside.
So those who say they’re using stevia, are really using Steviol Glycoside and/or Rebaudioside.
Different brands of “stevia” differ in their mixture of these two sweet chemicals, in the amount of purification, and in the additives which they use. But none of them is really the herb itself.
Hope this helps.