I thought I would show a little behind the scenes pictures.
This first one shows a piece of luann glued to the back of the drywall. This is where the toilet paper roller will eventually mount. Wood for the screws to bite into and hold.
That black area is the shower niche. The 2×8 blocks to either side, mounted by deck screws. Deck screws have higher sheer strength than other screws and will hold better than nails. This is where the shower grab bar will mount.