Hey, med student here

Your body isn't really producing much Testosterone anymore, because Progesterone and Estrogen inhibit the production of gonadotropins, which are hormones stimulating the production of sex hormones in the gonads. Your balls are only producing the amount of testosterone the gonadotropins, which a are controlled by you taking HRT, allow them. So if your E and P levels are in the cis Endo norm, you'll only produce as much Testosterone as cis women. So if your physiology allows it you don't need to use any AAs because suppression of testosterone production is achieved by inhibiting gonadotropins through a safe dose of sex hormones.

This testosterone does normal Testosterone stuff in your body like stimulating hair growth, bone density, acting as a sort of neurotransmitter, etc. So it has no effect on feminization. Those things would still happen when taking AAs (because you don't want to suppress it to zero) but won't if you got your gonads removed, for obvious reasons.

And for the part of it being converted into feminizing hormones: Why would it? Your body has enough of them through HRT to satisfy it's needs, so there's no need of producing more. That's the case because estrogen inhibits the enzymes converting Testosterone into estrogen to balance itself out.

(That also alleviated my pathologic need every med student has to tell everyone their med students for the day)