Most videos I've seen concerning kitless pen manufacture where the interior of the barrel would be visible or where the maker simply wanted the interior to be as clean as possible for holding and dispensing clean ink, the interior of the barrel was sanded and polished using a shop-made lapping tool. Typically a dowel or rod with a slot in it for holding the sandpaper or a bit of cloth with the polishing compound.
Makes more sense, in my mind, to make the required tool to your own custom specifications rather than to buy a kit full of tools that you'll likely never use 80% of them ... but I do like that the option is available!
If those are what I think they are Mal they are used with abrasive paste or lapping compound. The grit imbeds in the lap (soft copper or brass) and it polishes the harder metal in a bore. It won't work in a soft material like plastic.
You can take a dowel or rod and cut a slit in it to wrap fine sandpaper in to make a simple flap wheel. When spun with a drill with some favorite lube the paper will spin out and polish the side of the hole.
I know Micromesh make polishing swabs (something like a Q-tip) that should work especially for the end of the hole.
Slow typing on the phone gives you the answers by others as usual.
Skie, you don`t have to buy the whole set ... you can buy them individually of the size you need.
But I realize now that it would be more appropriate (for internal pen barrel polishing) to use a flap arrangement made from a slit dowel and a piece of abrasive paper, as Skie and Pete said.