After more exploring. I've concluded that the thin white layer is paint
that the wallpaper was put on top of. It has some kids scribblings on
it. So the brown/tan layer under that is the drywall cardboard, and
indeed, I picked one spot and found the gypsum under that. the house
was built in the 1950s.

So, should i be leaving the paint on, or trying to strip the paint
also? I intend to have professional painters paint the room when I'm
done. I decided to try stripping the paper myself to save some money.
Already the paint comes off at times, so there are sections of the wall
with paint and sections without. The drywall itself is coming out
undamaged, I'm pleased to say.