Basically, an AMAB body, (at least by default) produces minor amounts of estrogen and large amounts of testosterone, and can sense how many hormones it has in its body. If it detects a hormone deficiency, it will make more testosterone to counteract this. So taking estrogen will make the body make less testosterone to an extent, because it sees that your body already has hormones, and so won't make as much. One's body makes hormones their whole life, though it increases a lot during puberty.
Spiro is still taken, because testosterone is a more powerful hormone than estrogen, and does tend to override its effects if not suppressed. However, if estrogen levels are high enough, or surgery has been done to remove testes, the body won't be producing testosterone, and spiro is unnecessary.
(In the case of the latter, it's important that the person who went through surgery constantly take hormones of some form, as after this the body won't produce them naturally.)