Having been on HRT + Finasteride + occasionally Dutasteride for 9 months now, I can tell you that a lot of those tiny little fuzzy blond hairs will indeed go terminal. HRT can usually reverse about 7 years worth of hair loss. It will take about 3-5 years to do so, but it will recover a LOT of hair once all is said and done, especially if the loss is recent.
Any region on the head that is shiny, though, it won't regrow. That means that the hair follicles are dead. But if you can still see hairs there, but they've gotten thin and weak, that means that the hair isn't dead, just rendered weakened or inactive temporarily by DHT. And once the DHT is removed, the hair will indeed eventually regrow.
In my case, I started HRT with very thin hair in the front of my head. Pretty much the entire top of my head was see-through, and it had been getting thinner and thinner with every single year before that, starting when I was about 20.
And so that you have a good visual comparison, here's what my hairline looked like when I started HRT, and what it looks like now, 9 months in.


As you can see, there is quite a difference even in this short time. The hair started really regrowing around the 4-month mark, and has been thickening more and more with every single month since then. And right now, there's still a lovely crop of newly-terminal hairs right along the edges of my hairline. I still have a very defined "M" recession pattern, which I doubt is going to change, but it has been getting better. And the back of my head has also filled in a lot. I had a very noticeable half-bald spot back there before starting HRT, where now, although there's still some pink showing through, it's back to about the level of baldness that I had back there around age 23.
Basically, at first the hairs will pop back out as peach fuzz. A few months after that you'll notice that some of the peach fuzz along the edges of the hairline has gotten longer than the rest, and they look almost like small versions of normal blond hairs. A couple of months after that, where those longer blond hairs were, a few terminal hairs will be popping up, and there will be a line of shorter brown hairs along the edges of the hairline, that will continue to grow normally until they reach the same length as the rest of your head hair. This process will continue very slowly, progressing more and more, until about the 3-5 year mark, at which point it will even out. The amount of hair regrown, again, depends on how recent the loss has been, as well as just how low you're able to get your DHT levels, plus things like having a good diet.