Most drugs are just counted. Compounding pharmacists can make special pills at a doctor’s request. I had to have progesterone pills made specially for me at the beginning of my last pregnancy.
There’s a lot of filler in pills - The bulk is usually microcrystalline cellulose, plus whatever they want to color it with.
In as few words as possible, Propecia is a synthetic 4-azasteroid compound - in more words, it inhibits a specific enzyme.
As for the pharmacists, almost all standard Walgreens/Safeway/Costco type drugstores just count pills out of bulk bottles into your small prescription bottle. Outside of hospital pharmacies, it’s getting more and more uncommon to find compounding pharmacists, who are people that actually mix and prepare medications
Although monocrystalline cellulose is a very common filler , drug products usually contain more non-active (excipient) components than that. And I guess filler isn’t the best word - sometimes these components are needed to make a pill “behave” in the right way. A given pill might have lactose, or various starches, and some quantity of steric acid, or calcium or whatnot. “Long lasting”, slow release drugs will contain various amounts of release modifiers, such as Kollidon SR, ProSolv or Carbopol depending on the drug itself and how fast/slow they want the drug to be released from the tablet, and where in the body (stomach, upper GI, lower GI) it needs to be released to maximize absorption, etc.
A 1mg tablet contains, per tablet, 1mg of the active ingredient, and the rest of the weight is filler. In case you’re wondering, that 1mg is pretty accurate - potency assays need to be within (typically) 98.0-102.0% of that label claim in order for the drug to be released to the market The weight of the tablets also is very consistent, as is thickness and rate of dissolution, etc. The exact criteria changes per product.