Yes, it turns out I definitely didn't understand what you were asking.
I don't think it's known in an absolutely locked-down, every-mechanistic-detail way precisely how androgens increase hair loss in those subject to male pattern baldness but at least two mechanisms are involved: activation of the androgen receptor, and (of at least some importance but perhaps not much) an activity of 5a-androstanediol, a DHT metabolite, that increases hair loss.
In an individual with normal testosterone levels or only slightly increased testosterone levels, the 5a-reductase enzyme in the scalp yields greater effect at the androgen receptor at the scalp because total levels of hormone (testosterone plus DHT) are the same, but the percentage that is the more-active-for-any-given-concentration DHT form is higher. And because there is still room for further activity by the AR: it's not saturated yet or very close to it.
However in an individual with testosterone levels that are far above normal, then while it's true that the 5-AR enzyme will still have this effect, the importance will be a lot less because levels are so high that the receptors may be nearly saturated in either case.
And even besides that, the best that a 5-AR inhibitor can do is to cause all of the testosterone to still remain testosterone. Which itself is quite active at the androgen receptor. So there's no way for Propecia or any 5-AR inhibitor to stop this.
It is true that blocking 5-AR activity will result in blocking production of 5a-androstanediol but while perhaps that may be helpful, apparently it's not sufficient.
Many in bb'ing don't use 5-AR inhibitors during a cycle because of first not finding them effective and second because of concerns of loss of gains or feeling less "charged" mentally while using them.
The synthetic drugs you mention aren't affected by whether one uses a 5-AR inhibitor or not.
Basically, if planning to get the androgen receptors highly active in muscle, they're going to be highly active in the scalp as well, whether a 5-AR inhibitor is used or not.