What kind of chili peppers are these

When I go to the Korean supermarkets in LA, I usually see half an aisle just dedicated to 고추가루 in all kinds of forms (mild to spicy, fine to coarse grind) and colors. I don't know that there's any specific pepper than it is all about how sweet and mildly spicy 고추가루 is supposed to be. You could start from there to make your own by sun drying and crushing whatever peppers are available to you.

If you are looking for a substitute that you can buy at a western store, try crushed red pepper flakes (the kind you see in the shakers at pizza places). You're going to be adjusting for taste/spiciness anyway, so not much of a dealbreaker.

I am not sure if you are confusing roasting with charring but both are the same basically, though roasting the pepper by charring the skin would be a more accurate nomenclature.

Roasting brings smokiness to the pepper and softens its bite. It allows you to remove skin more easily. When you char the outside of dried peppers in a dry skillet with spices, then soak, it allows you to remove the pith more constructively than tearing it open and it adds depth to the flavor.

For fresh peppers they become somewhat less spicy, for dried peppers it allows them a more expressive flavor (think of roasted sesame or pumpkin seeds or coffee beans for a comparison). Roasted peppers add a different dynamic than their raw counterpart and you should definitely look into the difference (if you enjoy chipotles end adobo you already have a taste for them without knowing it).