1) Ply on floor does not have to be aterproofed.

2) Manufactured niches are waterproof. Install the niche and when you waterproof the cbu you'll seal the niche flange and cbu nice and tight.

3) It's certainly easier to buy a pre-made one and plop it in place. But you're limited by the offerings out there. If you want something different you can certainly frame your own. The niche I put in my shower is pretty wide, about 32" if I recall. Bottom opening is maybe 13" or 14" high, the top about 11 or 12" high. Wood framed, then covered in backer board. The roll-on membranes are fine, I used kerdi. then tile. Pitch the shelves for drainage.

Mongoct,
Thanks for that reply. Just to be sure if we make our own niche then after making it in wood, we put CBU on the inside back and sides, then roll on waterproof and then tile.
I guess we can use the same piece of CBU that we cut out of back.

Or you can just waterproof the sheetrock back from the wall on the opposite side of the framing, thereby saving that little bit more space.

Yes, typically wood, then cbu, then membrane.

Just read the installation instructions for the topical membrane that you chose to use. Some need reinforcing fabric at the corners, some do not.