Female birth control formulations contain both an estrogen and progestin. The desired biological effect is to stop ovulation, and involves a complex cycle between estradiol, progesterone, luteinizing hormone, and follicle stimulating hormone.
As far as what you will experience on HRT, the results will vary. I can tell you only what I experienced, which was a significant drop in libido and gender dysphoria within a week or so, and gradual physical changes, including softer skin, less skin oil, changes in body odor, and a gradual reduction in body hair.
HRT was a profound event for me. Prior to that, I was not sure if I would transition, or even if being transgender was a real thing, and not some bizarre fetish. After the first couple of weeks, I knew the answer to both of those questions.
With kindness,
Terri