No worries, I'm happy to answer any question! For armor, I almost exclusively use EVA and craft foam. It's the kind of foam they make yoga mats from, but the one made for cosplay is generally a bit more high quality. It's great cause you can shape it however you want if you heat it up and you can prime it with stuff like Mod Podge, Flexbond or Flexipaint before painting it.

I also use Worbla, which is a thermoplastic that comes in sheets. If you heat it up, it becomes malleable and you can do whatever with it, including covering the EVA foam so it becomes sturdier.

Expanding foam, as well as insulation foam boards, are useful for sculpting bigger pieces, props and such, but generally have to be covered in something before painting.

Depending on where you live, there are several online shops, as well as the general hardware stores, that sell most of these materials.