Transfem

Actually that understanding is totally being reevaluated. Anecdotally, my case is a good example, but far from unusual. I never took anti-androgens and my testosterone production significantly reduced once my estrogen levels started to raise. While still only using 1 patch, twice weekly for a few months, my testosterone dropped from around an average of 400-500ng/dL to 46ng/dL. My estradiol levels at that time were still in the low 100pg/mL range. Prior to my surgery I had moved up tp using 3 patches my to bring my estradiol levels to the low 200pg/mL on average and my testosterone dropped to around 10ng/dL.

I did misspeak a bit. You're body will consider one or the other to be your "primary" hormone and will prioritize production of that and deprioritize production of the other. But all bodies do produce both through various means. And in fact estradiol is essential to male sexual development. Just non-intersex, "average" bodies are not capable of producing as much of the opposite. But if the balance of hormones changes, most people's bodies will switch, but just be unable to maintain the balance if the imbalance was caused by supplementation.

There is not a hard male/female separation in any species with genders and never has been. Hormones, genetics, hormone intolerance, and many, many other factors play into what genitals we get at birth and what hormones we make and his efficiently we use them. The x/y chromosomes have a little to do with initial selection of "primary" hormone and thus genital creation in the womb, but if the body can't uptake testosterone effectively, then having a Y chromosome will not produce male genitals and the body will default to a more female configuration and produce estrogen primarily. It's one reason the "biological sex" fanatics don't actually want people to get their chromosomes tested. It would destroy their narrative to find out just his common it is for AFAB people to have Y chromosomes, vice versa, or totally different configurations outside of people traditionally classified as intersex.

My point on taking anti-androgens before knowing if you need them is because these are uptake inhibitors and your body us still producing the hormone, but you aren't using it, do your body produces more, etc. This means, until your testicles are removed, it's often too late to switch to estrogen-only if you start with anti-androgens. But it's still not understood how common it is to need them or not, and drug companies refuse to test any hormone therapy for trans people, which is why it's always "off-label" use, so studies rely primarily on existing data and volunteers and takes decades to compile.