yes, you could buy them loaded from the factory or you have the components - case , wad with primer and bullets for rifle cartridges. it was also common you get a bullet mould with your rifle and cast your own. you had in your relaoding stuff a powder dipper for measure the blackpowder. it was not terrible difficult for people who came from muzzle loader.

the use of percussion caps was possible but in allmost all cases you get factory mad wads in all possible caliber with a primer pill in the base. the primer pill was strong and very reliable.
the gun must be cleaned completely after shooting like a muzzle loader. thats the deal with blackpowder. of course, today with musket primer its a littlebit more for the needle but in the good old time with the pill and without any metal the needle would work sometimes thousends of shots.
anyway, making a new needle is part of the hobby.

"I asume oine makes a bunch of paper casings using a mandrel, loads them with several drams of powder, insert a wad with a percussion cap somehow located in the center, drop in an oz of shot, and seal the top. Easier said than done."

it is more less like you say

never see anything they use nitratet paper.