Estrogen is a term for many different molecules with some being produced by animals, others by plants and some are made in the laboratory. In humans, there are three - Estradiol E2, Estrone E1 and Estriol E3. The body normally produces E2 and that's the most potent estrogen in humans. E2 can be converted to E1 for storage however E1 is about a 10th the potency of E2. At some point, E2 is converted to E3 which is almost useless as it's a waste product that's eliminated from the body. If you are being given a human body compatible hormone in pill form, most likely it will be estradiol as it's really cheap and effective.
Depending on where you live, it is possible to get Estradiol Valerate which behaves a bit differently in the body however in the end, it's still estradiol E2.